The name of Malcolm Caldwell is remembered now by very few people: some friends, family, colleagues, and students of utopian folly. In the 1970s, though, Caldwell was a major figure in protest politics. He was chair of CND for two years, a leading voice in the anti-Vietnam war campaign, a regular contributor to Peace News, and a stalwart supporter of liberation movements in the developing world. He spoke at meetings all over the country, wrote books and articles, and engaged in public spats with such celebrated opponents as Bernard Levin.

The name of Kaing Guek Eav is, arguably, known by even fewer people, at least outside of Cambodia. Instead it is by his revolutionary pseudonym "Duch" that Kaing is usually referred to in the press. Duch is the only man ever to stand trial in a UN-sanctioned court for the mass murder perpetrated by the Cambodian communist party, or the Khmer Rouge, in the late 1970s. His trial on charges of crimes against humanity, grave breaches of the Geneva Conventions, and homicide and torture concerning thousands of victims, drew to a close in November. Justice has taken more than 30 years, but a verdict and sentence are expected sometime in the next few weeks.

Although their paths crossed only incidentally, the two men shared two main interests. They both had a pedagogic background: Caldwell was a history lecturer at the School of Oriental and African Studies (SOAS), University of London, while Duch, like many senior Khmer Rouge cadres, started out as a schoolteacher. And they both maintained an unbending belief in Saloth Sar, the leader of the Khmer Rouge revolution, who went under the Orwellian party title of Brother Number One, but was known more infamously to the world as Pol Pot. It was an ideological commitment that would shape the fate of both men and they held on to it right up until the moment of death – in Caldwell's case, his own, for Duch, the many thousands whose slaughter he organised.

In each circumstance, the question that reverberates down the years, growing louder rather than dimmer, is: why? Why were they in thrall to a system based on mass extermination? It's estimated that around two million Cambodians, more than a quarter of the population, lost their lives during the four catastrophic years of Khmer Rouge rule. What could have led these two individuals, worlds apart, to embrace a regime that has persuasive claim, in a viciously competitive field, to be the most monstrous of the 20th century?

When Caldwell appeared at SOAS for an interview in the late 1950s, the senior faculty thought that they had landed one of the academic stars of the future. Caldwell, who took his PhD at Nottingham University, had gained a reputation as a bright young talent and, according to college legend, he presented himself as a sober scholar.

"So they hired him," recalls Merle Ricklefs, a former SOAS colleague and now a history professor at the National University of Singapore. "Then he showed up for lectures and suddenly he was this Scottish radical with long hair, looking unkempt, and they felt as though they'd been betrayed.

"I thought he was actually a very good economic historian," says Ricklefs, who remembers "an extraordinary character… very ideologically committed". He was also struck by his warmth and good manners. As a young American, who dressed in conservative fashion, arriving in England during the height of the Vietnam war, Ricklefs expected to be greeted with a certain amount of antipathy, but he found Caldwell to be "always cordial. Always looking slightly dishevelled and revolutionary, but never the slightest hint of discourtesy."

The picture of a friendly, if rather unconventional character, is confirmed by others who knew him. Professor Ian Brown was Caldwell's successor at SOAS and he was also his former student. "He was well liked – I suspect not by the SOAS hierarchy," says Brown, "but certainly loved by students and colleagues."

He describes a "skinny, somewhat emaciated, rather scruffy character who, bizarrely, always used to wear a suit – though it was clearly a suit that had been bought in the 1950s equivalent of Oxfam and not seen too many dry cleaners." Caldwell never hid his politics from his students, indeed he made a point of proselytising to them. One of his protégés was Walter Easey, who, according to Easey's obituarist, Caldwell converted to "a fierce and angry communism". But to Professor Brown, "he was a gentle person, quietly spoken, and very tolerant of opposing views. He treated everyone well. He was very encouraging and a really inspiring teacher."

Both Brown and Ricklefs use the same word to describe this well-travelled, extremely well-read and highly intelligent man: naive. SOAS, says Brown, was a college whose standing and ethos rested upon sound empirical study. "Everyone else in the history department went off every summer to the archives in Rangoon, Baghdad, etc, and got deep inside the data. Malcolm didn't. He was a man with very clear theoretical and ideological views and the empirical basis didn't seem to worry him hugely."

It's not that Caldwell was lost in bookish abstraction, for he did visit the various communist regimes he extolled. It was more that when he got there he was all too willing to accept state propaganda as verified fact. For example, he praised the "magnitude of the economic achievements" of Kim Il-Sung's impoverished North Korea and, returning from a trip to the highly secretive state, he wrote that the country was "an astonishing tribute not only to the energy, initiative and creativeness of the Korean people, but also to the essential correctness of the Juche line". "Juche" was the mixture of ultra-nationalism and self-reliance on which Kim built his monumental personality cult. About the totalitarian surveillance and ruthless political repression, Caldwell said nothing.

Although academic traditionalists may have disapproved of Caldwell's slanted scholarship, many idealistic students were inspired by his lectures. Tariq Ali, who became famous as a 1968 student leader, recalls going to see him talk on southeast Asia when Ali was at Oxford. They soon got to know each other and in the summer of 1965 went to a peace conference together in Helsinki. "We had to fly to Moscow," says Ali, "then there was a train, via Leningrad as it was then, to Helsinki. We talked a lot and became very friendly. It was later on that his Cambodian deviation was a bit off-putting. And he could never completely explain it."

At one time, the pair discussed opening a Vietnamese restaurant as a sort of act of antiwar gastro-prop. "He would say that after a few drams," Ali recalls. "He was a great whisky drinker. He was also a great cricket fan and an early Scottish nationalist."

Cricket is mostly followed in Scotland by the upper classes, but Ali got the impression that his old friend came from a middle-class background. His Wikipedia entry states that he was the son of a miner. "You know," says Ali, "we never bothered about these things. We were so totally immersed in politics and the state of the world, we never really talked about each other, our personal lives or social backgrounds."

In seeking to understand why this idealistic Scotsman became a cheerleader for Pol Pot, it would be wrong to consign him to the maverick margins. A member of the Labour Party, he stood as a candidate in the 1977 local elections in Bexley. John Cox, who followed in Caldwell's footsteps as chair of CND, is adamant that there was nothing out of the ordinary about his predecessor's politics. "He was well in the mainstream of what I would call generally progressive liberal thinking," says Cox.

This idea that support for the most illiberal systems of government is all part of the liberal tradition is one of the more bemusing aspects of progressive politics. But the missing factor in the equation is the view that the United States of America is the ultimate villain. The background to the brutality visited on Cambodia was the brutality visited on Vietnam by US forces.

Although the Vietnam war was more complex than is often acknowledged (the tensions between North and South, for example, long predated the war), the Americans essentially inherited France's colonial conflict. But they fought it in the context of the Cold War. As much as US administrations may have seen the battle as one between communism and the free world, to the majority of Vietnamese it was a liberation struggle.

In an effort to close down North Vietnamese supply lines to the South, the US also launched a devastating bombing campaign on neighbouring Cambodia. Instead of winning the war in the former, it served only to destabilise the latter. To make matters worse, an American-supported coup put in place the corrupt government of Lon Nol in Phnom Penh. So there was a tendency among many anti-war protesters to see the Khmer Rouge as just another national liberation movement, fighting to escape from under the American yoke.

One man who observed the truth up close, four years before the Khmer Rouge came to power, was a French ethnologist called François Bizot. In 1971, while out researching Buddhist practices, he was captured in the Cambodian countryside by Khmer Rouge insurgents. He was held captive with scores of Cambodian prisoners at the M-13 prison camp, a precursor to the 196 santebal (secret police) offices that were set up after the Khmer Rouge seized power. The head of the camp, and the Frenchman's tireless interrogator, was Duch.

Bizot wrote about the encounter in a remarkable memoir called The Gate. After three months, during which he was shackled and repeatedly accused of being an American spy, he was suddenly released – all the other prisoners were executed. So relieved was the Frenchman that he asked Duch if he would like a gift. His jailer thought for a while and then replied, "with the look of a child writing to Father Christmas, 'The complete collection of Das Kapital by Marx.'"

Three days before Christmas in 1978, Malcolm Caldwell received an early present. On the final day of a two-week tour of Cambodia, he was told that he would meet Pol Pot. This was indeed a rare privilege. Unlike most other communist leaders, Pol had not created a personality cult. There were no posters of him. He was seldom seen or quoted. Many Cambodians had not even heard of him. Only seven westerners were ever invited to what had been renamed Democratic Kampuchea. And Caldwell was the first and only Briton.

There were several reasons why Caldwell had been received in Phnom Penh. He was on good terms with China, Cambodia's main ally in the region. There were also growing tensions between Cambodia and its larger neighbour Vietnam and, fearful of an invasion, Pol Pot was belatedly attempting to improve Kampuchea's image abroad. Most of all, while other supporters had wavered, Caldwell had remained steadfast. Only months before, he had written an article in the Guardian, rubbishing reports of a Khmer Rouge genocide. He cited Hu Nim, the Kampuchean Information Minister, who blamed the deaths on America. Caldwell was unaware that Hu had himself already been tortured to death in one of Pol Pot's execution centres. Such killings that the Khmer Rouge had committed, argued the peace activist, were of "arch-Quislings who well knew what their fate would be were they to linger in Kampuchea".

Travelling with Caldwell were two American journalists, Elizabeth Becker and Richard Dudman. Becker had been a foreign reporter in Phnom Penh during the civil war that brought the Khmer Rouge to power. She knew the terrain, and had been to Thailand to talk to refugees. She and Caldwell argued endlessly about the true nature of the situation.

"He didn't want to know about problems with the Khmer Rouge," she says. "And that carried over to not wanting to know about problems between Cambodia and Vietnam. He was stuck in '68 or something."

Yet for all their disagreements, she liked Caldwell. "He was a lovely man, very funny, very charming," she says. "A real sweetie. He was also very homesick for his family and he said he'd never spend another Christmas away from them."

According to Becker, Caldwell had not read François Ponchaud's Cambodia: Year Zero, the book that first catalogued the Khmer Rouge genocide. A friend of François Bizot, Ponchaud was a Catholic missionary who was in Phnom Penh when the victorious Khmer Rouge army marched into town. His book became required reading for anyone interested in what was happening in Cambodia. "The fact that Malcolm, a professor, had not read it before he went, that I couldn't believe," says Becker. "I think it was almost ideological that he didn't read it."

It's perhaps not that strange that Caldwell had neglected to read Ponchaud, given that he had already dismissed the Frenchman's credibility in print. He based his damning opinion on a brief extract of Year Zero which the Guardian had published and a critique of the book by the American academic, Noam Chomsky. An icon of radical dissent who continues to command a fanatical following, Chomsky had questioned the legitimacy of refugee testimony that provided much of Ponchaud's research. Chomsky believed that their stories were exaggerations or fabrications, designed for a western media involved in a "vast and unprecedented propaganda campaign" against the Khmer Rouge government, "including systematic distortion of the truth".

He compared Ponchaud's work unfavourably with another book, Cambodia: Starvation and Revolution, written by George Hildebrand and Gareth Porter, which cravenly rehashed the Khmer Rouge's most outlandish lies to produce a picture of a kind of radical bucolic idyll. At the same time Chomsky excoriated a book entitled Murder of A Gentle Land, by two Reader's Digest writers, John Barron and Anthony Paul, which was a flawed but nonetheless accurate documentation of the genocide taking place.

We can never know if Caldwell would have taken Ponchaud more seriously had Chomsky not been so sceptical, but it's reasonable to surmise that the Scotsman, who greatly admired Chomsky, was reassured by the American's contempt. In any case, the 47-year-old Caldwell arrived in Cambodia untroubled by the story that Ponchaud and others had to tell. In fact, he had just completed a book himself that would be posthumously published as Kampuchea: A Rationale for a Rural Policy, in which he wrote that the Khmer Rouge revolution "opens vistas of hope not only for the people of Cambodia but also for the peoples of all other poor third world countries".

With Dudman and Becker, Caldwell was escorted around the country to a series of staged scenes. Alarmed by the changes she saw and frustrated by what she was not allowed to see, Becker grew increasingly combative with her hosts. "It was so clearly awful," says Becker. "One of the problems was the absence of what I saw. The absence of people. And that's a different kind of proof to 'I don't see any people being executed.'"

Caldwell was not unduly bothered. "He preferred to stay in the car and laugh at the clumsy photo opportunities prepared for us," Becker wrote in her book on Cambodia, When The War Was Over.

"He'd travelled to other communist countries," she tells me now, "and he knew exactly what the PR routine was and he thought that all governments do PR. He did not know Cambodia, and he didn't speak the language. If you don't speak the language, don't know the country, you can edit out a little more easily."

At the end of the tour, the party returned to Phnom Penh, which Dudman described as "a Hiroshima without the destruction, a Pompeii without the ashes". They stayed at a guest house near the centre of Monivong Boulevard, one of the empty city's main thoroughfares. Close by was the secret facility of Tuol Sleng, a former school that had been turned into an interrogation centre. Known as S-21, Tuol Sleng specialised in gaining confessions through torture. Between 14,000 and 16,000 prisoners – men, women and, most hauntingly, children – passed through its gates, including Hu Nim. Only seven survived. It was run by Duch.

Nowadays Tuol Sleng is a genocide museum, and an established part of the southeast Asian tourist trail. Although they were intent on erasing history, Pol Pot and his senior cadres were obsessed with the accomplishments of the 12th-century Hindu dynasty that built the temple complex of Angkor Wat and constructed elaborate dam and irrigation systems. They considered their own contribution to Khmer culture to be of a similar, if not greater, significance. It speaks eloquently of the Khmer Rouge's achievements that, while Angkor Wat remains the country's main tourist attraction, the next most popular sights for visitors are Tuol Sleng and the Killing Fields at Choeung Ek, where the prisoners from S-21 were taken to be "smashed" – usually with an ox-cart axle. A ghost town under the Khmer Rouge, Phnom Penh is now a bustling, sprawling city, dense with people and commercial activity. In May 1975, one month after the Khmer Rouge evacuated the capital, the Swedish author Per Olov Enquist wrote: "The brothel has been emptied and the clean-up is in progress. Only pimps can regret what is happening."

If that was blatant wishful thinking, it's an unpalatable truth that the pimps have returned. A potent mix of Developing World poverty, cheap flights and sexual licence has made Cambodia a magnet for sex tourists and paedophiles. The upmarket hotels around the riverside are full of western and Japanese businessmen, and a certain kind of furtive middle-aged traveller, stubble-chinned and plump-stomached, is a conspicuous presence in the bars and clubs frequented by young and under-age prostitutes.

Cambodia has just two seasons: wet and dry. It either rains or it doesn't, a binary climate that may have helped shape the Khmer Rouge Manichean view of the world – revolutionary or counter-revolutionary, insider or outsider, good or bad. It was the dry season when I visited in late November, and a cooling wind blew through the hot, polluted streets. At first sight, Tuol Sleng's large courtyard, lined with coconut palms, provides welcome respite from the noise beyond. A respectful silence is maintained by visitors, including groups of western backpackers, with their cameras and guidebook glaze. The three-storey buildings have been left pretty much as they were abandoned in 1979, slightly dilapidated with jerry-built cells, barbed-wire fences and medieval instruments of torture. The effect is to transport the visitor not just back in time, but also into the reptilian depths of the imagination, a merciless place of zero compassion.

In the courtyard of the prison is a poster listing the rules of the camp. None of them makes for pleasant reading. For example, number 2 states in an imperfect translation: "Don't try to hide the facts by making pretexts this and that. You are strictly prohibited to contest me." It vividly articulates the mentality that shaped S-21, and indeed Kampuchea beyond, the relentless determination to remove every option from the prisoner – and citizen – to reduce them to absolute compliance. But perhaps the most disturbing is number 6: "While getting lashes or electrification you must not cry out at all." Denied every human and judicial right, the inmates were also refused the one prerogative of the tortured: the right to express pain.

I visited the archive on the second floor of the building, where some of the 4,000 files the Vietnamese discovered are housed. Here, I was brought the "confession" of John Dewhirst, a 26-year-old teacher from Newcastle who was captured in 1978, while sailing with friends through the Gulf of Thailand. Intercepted by a Khmer Rouge patrol boat, they were placed in S-21 and tortured over the course of a month. As the weeks passed, Dewhirst made a series of ever more bleakly surreal confessions. They start out as straightforward biography – he explains that he had studied at Loughborough University. Then he admits to being a CIA agent, recruited at Loughborough where the CIA, he is made to say, maintains one of its covert training bases. It "was housed in a building disguised as the Loughborough Town Council Highways Department Surveyor's Office". He also reveals that his father is another CIA agent, using the cover of "headmaster of Benton Road secondary school". Dewhirst was murdered by the Khmer Rouge in 1978.

S-21 was not concerned with the truth. Its only aim was to derive the fullest possible confession in accordance with party requirements. In his book Voices From S-21, the historian David Chandler quotes Milan Kundera's phrase (used to describe the Soviet bloc secret police) of "punishment seeking the crime" to sum up the prison's project. To this end, the most depraved techniques – electric shocks, rape, the forced eating of excrement, medical experimentation, flaying, and lethal blood extraction – were employed. It's hard to comprehend that these agonies were not just formalities, they were preliminaries. It wasn't a question, on arriving at the prison, that an inmate would be lucky to get out alive. He or she would be lucky to get out just dead. A guidebook for interrogators clarified the issue: "The enemies can't escape from torture; the only difference is whether they receive a little or a lot."

The precise level of punishment was decided upon by Duch. If the confession was not sufficiently elaborate, the punishment was increased. In these situations Duch impressed upon his staff that "kindness is misplaced". Some interrogators were more disposed to brutality than others. And some were simply demented sadists. The most sadistic of them all went by the name of Toy, a pitch-black irony that his English-speaking victims were in no position to appreciate. In recent testimony, a prison guard recalled that one of Dewhirst's party (either the young teacher himself or the New Zealander or Canadian travelling with him) was burned alive in the street. The order that they be incinerated came directly from Pol Pot.

Just a few months after that grisly murder, Caldwell prepared himself to meet the man who commissioned it. The Scotsman knew little or nothing of Dewhirst's fate. Instead his mind was on agrarian revolution. Caldwell believed that the world was accelerating towards a global famine and that the answer was Developing World self-sufficiency. But Cambodia was a strange place to test his theory. As Professor Ian Brown notes: "This is a part of the world that historically had not been a food-deficient area, so you wouldn't go looking for a crisis there. Again, that seems to indicate a more fundamental flaw in his approach: he comes at it with a theoretical position. And therefore he'd search for an argument, not necessarily evidence, that will sustain that."

In Pol Pot, Caldwell found someone with an argument that suited his purposes. Pol's plan was a massive increase in rice production to finance Cambodia's reconstruction. It required collectivisation and slave labour, though Caldwell preferred to see the effort in terms of spontaneous revolutionary spirit. In the event, owing to the shortage of technicians and experts (who were killed as class enemies) and lack of peasant support, production fell well short of targets. But terrified of underperforming, regional commanders still sent their designated contribution to be exported. The result was the opposite of self-sufficiency: famine. Unable to accept the shortcomings in his plans, Pol instead blamed spies and counter-revolutionaries, and that meant that, in the absence of rice, spies and counter revolutionaries had to be produced. The network of torture camps was the only area of Democratic Kampuchea's infrastructure that met its targets.

Of these dreadful facts, Caldwell remained ignorant on the Friday morning in Phnom Penh that he was taken in a Mercedes limousine to see Pol Pot. The setting for the meeting was the former Governor's Palace on the waterfront, built during the French colonial period. In a grand reception room replete with fans and billowing white curtains, the two men sat down and discussed revolutionary economic theory.

Becker had met Pol Pot earlier the same day, and in When the War Was Over she writes: "He was actually elegant, with a pleasing face, not handsome but attractive. His features were delicate and alert and his smile nearly endearing."

The perennially shabby academic and the fastidious dictator must have made for an odd couple. In any case, Caldwell left the meeting a happy man. He returned to the guest house he was sharing with Becker and Dudman, full of praise for Pol Pot and his political outlook. "We went over stuff," says Becker. "He thought he had had a good conversation. He had avoided at all costs any discussion of Vietnam. And he was looking forward to going home."

That night they all had dinner together and afterwards Dudman went to his room. Becker and Caldwell "stayed at the table to have our last argument about Cambodia". He took the longer view and said that the revolution deserved support. She, on the contrary, was even more convinced of the refugees' testimonies. "That night," she writes, "Caldwell tried once more to get me to change my mind."

Becker went to bed at 11pm and was woken a few hours later by the sound of what she took to be dustbins. Coming to her senses, she realised there were no dustbins in Phnom Penh. What she had heard was gunfire. She opened her bedroom door to see a young man pointing a pistol at her. He was wearing two bands of ammunition and carrying an automatic rifle over his shoulder. She begged him not to shoot and locked herself in her bathroom.

Meanwhile Dudman had woken up and, looking out of his window, saw a file of men running along the street. He knocked on Caldwell's door. The two men spoke briefly and then a heavily armed man approached. The man shot at the floor and Dudman ran into his room. Two shots were fired through his door. The two Americans remained hiding in their rooms for the next hour before an aide arrived and told Becker to stay where she was. Almost another hour passed before she was allowed to come out. Caldwell, she was told, had been shot. He was dead.

The Extraordinary Chambers in the Courts of Cambodia (ECCC) are located in a large, purpose-built court on the dusty outskirts of Phnom Penh. During the course of last year, hundreds of Cambodians made the trip out from the city and in from the countryside to bear witness to a long-overdue reckoning.

The lone defendant in the trial is a slim, well-preserved 67-year-old with small, sensitive eyes. With his thick grey hair and concentrated expression, he looks like a sprightly grandfather, a little stiff and formal, but sufficiently attuned to the contemporary world as to be smartly dressed in a Ralph Lauren shirt or, on another occasion, a cream cashmere roll-neck sweater. A giant bullet-proof glass screen divides the court from the auditorium, where 500 or more people sit watching the proceedings. Centre stage is Duch (pronounced "Doik" in Khmer), seated with his back to the audience. To his left is a bank of lawyers, and behind them in the corner the relatives of victims. In front of the defendant sit the judges, on an imposing two-tier stand. Ten years, some 400 staff, a dozen judges, a battery of international lawyers, an ongoing legal wrangle, and many millions of pounds is what it has taken to put Duch on trial.

Following Caldwell's murder, four guards assigned to the tourist's protection team were arrested and taken to S-21. Owing to the importance of their alleged crime, the commandant of the prison was instructed to head their interrogation. So the stories of Caldwell and Duch came together at the inevitable point of a torture camp. Here, amid bestial squalor, is where the liberation dream ended.

Two of the "confessions" made by guards referred to in their S-21 files as "the Contemptible Met" and "the Contemptible Chhaan", outline a baroque conspiracy involving many other people. The Contemptible Chhaan gives an explanation for the murder: "First, we were attacking to ruin the Party's policy, to prevent the Party from gathering friends in the world… And in attacking the guests on this occasion, we would not attack them all. It would be enough to attack the English guest, because the English guest had written in support of our Party and the Kampuchean people for a long period of time already… Therefore, we must absolutely succeed in attacking this English guest, in order that the American guests would write about it."

Whether this was yet another example of innocent men implicating other innocent men, it's impossible to know. Certainly there must have been some kind of in-house involvement, as the guests were guarded. But who instructed the guards, and why they did so, remains a subject of speculation. Some argue that the Vietnamese were behind the killing, others that it was a function of an internal party struggle.

Caldwell's brother, David, wrote a letter to the Guardian, expressing his belief that "Mal" had "discovered the truth about the Pol Pot regime" but "dared not admit this to either Becker or Dudman". This seems unlikely. David Chandler told me that he once met the translator of the meeting between Caldwell and Pol Pot, who remembered a very pleasant exchange conducted in a spirit of enthusiastic agreement. If that anecdote suggests Caldwell died a dedicated Pol Potist, it tells us little about Pol, a man for whom the word "inscrutable" might have been invented. As his deputy, Ieng Sary, later recalled: "Pol Pot, even when he was very angry, you could never tell. His face… his face was always smooth. He never used bad language. You could not tell from his face what he was feeling. Many people misunderstood that – he would smile his unruffled smile, and then they would be taken away and executed."

But why would he seek international support by killing one of his few remaining friends from abroad? It makes no sense. "Don't apply rational thinking to the situation," Becker cautions. "It was crazy. Crazy. Malcolm's murder was no less rational than the tens of thousands of other murders." The journalist Wilfred Burchett claimed to have seen a Cambodian report not long after Caldwell's death, which stated that he "was murdered by members of the National Security Force personnel on the instructions of the Pol Pot government". Burchett theorised that Caldwell had changed his mind about the regime, but all the available evidence indicates otherwise. In the end, Becker's conclusion seems to be the most satisfactory: "Malcolm Caldwell's death was caused by the madness of the regime he openly admired."

The confessions of Caldwell's alleged killers were completed on 5 January 1979. Either that day or the following one, the four men were bayoneted to death in the prison itself. They were very possibly the last killings to take place at S-21. On 7 January, the Vietnamese army arrived in Phnom Penh, and Pol Pot and his associates fled into the jungle.

The contrast between the care taken to observe Duch's legal and human rights and the indifference with which he dispatched his victims is lost on no one. But as Philippe Canonne, one of the lawyers representing the relatives of the victims, said of the urge to inflict on Duch what he had meted out to his prisoners: "We must give voice to this sentiment, but then have the strength to transcend it."

It's this sort of resolution that has made the trial a legal landmark in a nation that has had little experience of the rule of law. That it was ever staged at all is a major accomplishment. For 20 years after the Vietnamese invasion, Duch lived at liberty. At first he followed the bulk of the Khmer Rouge into exile on the border with Thailand. After the fall of the Khmer Rouge, the US and China refused to accept the Vietnamese puppet government installed in Phnom Penh. In a shameful version of the principle that my enemy's enemy is my friend, they instead persuaded the UN to recognise a coalition resistance movement, of which the Khmer Rouge formed the major player. Thus Pol Pot was afforded the support of China, the protection of Thailand, and the indirect recognition of the United States.

For two decades the Khmer Rouge waged guerrilla warfare against the government in Phnom Penh. Then, in 1997, Pol Pot was placed under house arrest by his fellow Khmers Rouges. He died peacefully in his sleep on 15 April 1998. A year later the photojournalist Nic Dunlop found Duch working for a Christian relief agency. An interview was duly published and Duch handed himself in to the Phnom Penh authorities.

In theory, the trial is a joint effort between the UN and Cambodia, but the effort has been all the UN's. The Cambodian People's Party, which has ruled since Pol Pot was overthrown, is led by onetime Khmer Rouge members who, under threat of purging, had defected to Vietnam. One of these is Hun Sen, a former revolutionary soldier, who has been prime minister since 1985. His government was accused by Amnesty International of widespread torture of political prisoners, using "electric shock, hot irons and near suffocation with plastic bags". And for many years, senior former members of Pol Pot's government lived under protection in Cambodia, some with family links to the government. So there were several reasons why a major trial with international media coverage was potentially embarrassing or inconvenient.

After much pressure, in November 2007 the Cambodians finally arrested the four most senior surviving Khmer Rouge leaders: Nuon Chea, Ieng Sary, Ieng Thirith and Khieu Samphan. Their trial is scheduled to start in 2011, though few observers will be surprised if it is indefinitely delayed. All of them claim ignorance of any wrong-doing. Perhaps the most galling example is a long letter of evasion and self-justification that Khieu Samphan, Pol Pot's chief ideologue, wrote to Cambodian newspapers in 2001. "I do not see any importance in bringing up this tragic past. We would be better off to let everyone be at peace so that all of us can carry on our daily tasks… I tried my best for the sake of our nation's survival, so that we might enjoy development and prosperity like other nations. I am so surprised that this turned out to be mass murder."

In one form or another, this exculpation has been used over and again by the supporters of communist revolutions, from the Russian via the Chinese through to the Cambodian. Each new manifestation commanded the fervent advocacy of a new generation of radicals. Sooner or later the grim reality was revealed, which, paradoxically, only raised the hope that the next version would get it right. As the French philosopher Jean-François Revel has remarked: "Utopia is not under the slightest obligation to produce results: its sole function is to allow its devotees to condemn what exists in the name of what does not."

Somehow the link between Marxist-Leninist ideology and communist terror has never been firmly established in the way, for instance, that we understand Nazi ideology to have led inexorably to Auschwitz. As if to illustrate the point, earlier last year the ECCC announced that Helen Jarvis, its chief of public affairs, was to become head of the victims unit, responsible for dealing with the survivors, and relatives of the dead, of S-21.

Jarvis is an Australian academic with a longterm interest in the region, who was recently awarded Cambodian citizenship. She is also a member of the Leninist Party Faction in Australia. In 2006 she signed a party letter that included this passage: "We too are Marxists and believe that 'the ends justify the means'. But for the means to be justifiable, the ends must also be held to account. In time of revolution and civil war, the most extreme measures will sometimes become necessary and justified. Against the bourgeoisie and their state agencies we don't respect their laws and their fake moral principles."

Jarvis refused to speak to me about these matters. But Knut Rosandhaug, the UN's deputy administrator for the tribunal, said that the administration "fully supports" her. In this sense, although she was never a Pol Potist herself, Jarvis shows that the spirit of Malcolm Caldwell has survived the last century. It lives on in the conviction that the ends justify the means, and in the manner that liberal institutions can house the most illiberal outlooks.

The means, of course, always become the ends. Duch or someone like him is the method and the madness, the process and the final product. At least the man himself claims to grasp what continues to elude too many who should by now know better. In his deposition to the court, he said: "I clearly understand that any theory or ideology which mentions love for the people in a class-based concept is definitely driving us into endless tragedy and misery."

The following day, his lawyer, Kar Savuth, asked that Duch be acquitted and set free.

Caldwell didn't trouble himself with the means in Cambodia. He was too focused on an imaginary end, which meant that he never glimpsed the deadly real one approaching.

"He may have been starry eyed," says John Cox. "But we all do that. Even my local football team I support long after they've been destroyed match after match. It's a human failing."

A few days after Caldwell's murder, a testimonial was published in the Guardian.

"Caldwell," the writer said, "was an irreplaceable teacher and comrade whose work will undoubtedly suffer the customary fate of being better appreciated after his death."

As it turned out, history has forgotten Caldwell. But the amiable apologist for tyranny should be remembered, if only so that we don't forget history.★