It is a tree like many others. Neither old nor distinctively tall, it has rough, brittle bark and a pile of bricks at its base. Sticks of incense left unlit between the bricks might mark the tree out elsewhere, but not in Southeast Asia, where trees are the infrastructure of an Animist spirit world. What sets the tree apart here is a sign that reads, “KILLING TREE AGAINST WHICH EXECUTIONERS BEAT CHILDREN.”

You imagine it, while you read the sign. An infant still soft with baby fat is held by its legs and swung against the trunk of the tree repeatedly, until its skull cracks. The executioner is a country boy of seventeen without an education. A thorough indoctrination has failed to prepare him for the job of killing infants, but the boy obeys out of fear, and it is his horror you feel most keenly, after he has discarded the child with its mother in a mass grave. You want the words on the sign to be nonsensical – just words, like Noam Chomsky’s “Colourless green ideas sleep furiously” – because what they point to is black, bilious, mad. The tree on the other hand is ordinary, and it is its ordinariness that starts to work on you after a while, when you reach out to rub the rough bark, wondering how genocide could leave its blunt instrument unmarked.

You enjoy a moment of satisfaction when you recognise bone fragments and a tooth nearby, on a cordoned-off patch of dust. You were looking out for the tooth. It made an impression on a friend and you feel that it ought to make an impression on you, but squatting down with your camera to frame it beside broken femurs, you’re left awkwardly, disappointingly unmoved.

Around you, people are milling between open graves, carrying on in familiar roles. The holidaymakers are still holidaymakers, amused and interested by a foreign place. They stop for photographs at the memorial’s centre, in front of a tall stupa that resembles Theravada Buddhism’s cremation towers. Over 5,000 human skulls are stacked in well-ordered piles inside it, with neat, anthropological labels like “SENILE FEMALE KAMPUCHEAN OVER 60 YEARS OLD,” or “JUVENILE MALE KAMPUCHEAN FROM 15 TO 20 YEARS OLD,” but the skulls all look the same to you.

The holidaymakers are arranging themselves for a group portrait. What are they commemorating, you wonder, as they smile mechanically at the sound of “Cheese!” A group of boisterous Cambodians are also making their way through the field. Choeung Ek is an outing on a day off work, and the young men whoop as they pass excitedly from the Baby Tree onto a Perspex box containing miscellaneous bones.

A visitor centre is set off to one side, in a part of the memorial visitors take in first or save for last. The afternoon is oppressively hot, but it is context you are thirsty for as you make your way over, leaving behind Choeung Ek’s miasma. Inside, a clumsily translated introduction celebrates the depths of Cambodia’s debasement. “With a tremendous Memorial Stupa of the Choeung Ek Genocidal Centre,” it reads, “we imagine that we are hearing the grievous voice of the victims who were beaten to death by the killers with canes, bamboo stumps or head of hoes, and were stabbed with knives or swords. We seem to be looking at the horrifying scenes and the panic, the stricken faces of the people who were dying of starvation, forced labour or torture without mercy upon the skinny body and they died without giving the last words to their kith and kin. How bitter were they when seeing their beloved children, wives, husbands, brothers or sisters were seized and tightly bound and taken to the mass graves while they were waiting for their turn to come and share the same tragic lot?”

There are other signs. You peer at them myopically, craning forward in the dim light. Some itemise sandals and clothing that belonged to the deceased, others illustrate murder techniques. The only sign that shocks you is a simple flowchart, outlining the structure of Democratic Kampuchea. Pol Pot is at its top. Below him are two people; below them, another three. A total of just six people. The sign poses a question the memorial makes no attempt to answer: how?

The Communist regime that controlled Cambodia between April 1975 and January 1979 was known as Democratic Kampuchea (DK). The revolution it sponsored swept through the country like a forest fire or a typhoon, and its spokesmen claimed that ‘over two thousand years of Cambodian history’ had ended. So had money, markets, formal education, Buddhism, books, private property, diverse clothing styles, and freedom of movement…On a national scale, it is conservatively estimated that between April 1975 and January 1979 over one million people—or one person in seven—died as a direct result of DK policies and actions. These included overworking people, neglecting or mistreating the sick, and giving everyone less food than they needed to survive. Perhaps as many as one hundred thousand people were killed outright as enemies of the revolution.

David Chandler, A History of Cambodia

Academics have blessed visits to Cambodia’s Killing Fields with two names. The first is thanatourism, after the ancient Greek personification of death, Thanatos. Unsurprisingly, the name hasn’t stuck, and dark tourism – a simpler term, with broader associations – is more frequently used. There is uncertainty too over what activities dark tourism should include. Choeung Ek is undeniably dark, as is Auschwitz, which was visited by over 1.4 million people last year, but are our fascinations with natural disasters, battlefields and tombs equally morbid? Is James Dean’s grave different to Jim Morrison’s, because he died violently? Are museums also a type of dark tourism, in spite of the ways they mediate the atrocious, brutal or macabre? Why, in short, do we go, especially when we’re on holiday, and should we go at all?

There is an ongoing argument over whether dark tourism is a postmodern phenomenon, born out of 24-hour, calamity-fixated news, or something much older, with antecedents as far removed as Rome’s gladiatorial games. Phillip Stone, who has his own Institute for Dark Tourism Research at the University of Central Lancashire, was the first to articulate the latter argument. He points to public executions in Victorian London, which were explicit spectacles viewed from grandstands, as well as Madame Tussaud, who toured with her Chamber of Horrors from 1802. England’s first guided tour “took in the hanging of two convicted murderers” in 1838, at a time when people paid to watch floggings and visit morgues, and all of this morbid curiosity coincided with the Victorian invention of the tourism industry, epitomised by Thomas Cook and his traveller’s cheques.

Academics argue less about why we go. Dark tourists want to consume death, they say – to sniff its rot before escaping smugly back into sweet-smelling life. If they’re right, we are voyeurs at best and implacable sadists at worst, and it is on this conclusion that Phillip Stone hangs his for-hire sign: he wants to develop dark tourism’s sites in ways that allow us to observe without indulging in schadenfreude. It is a worthy goal, but lumping together tombs and concentration camps won’t help him reach it.

There are straightforward differences between commemorating sad but otherwise routine deaths – like the plane crash that kills a young star – and systematic murder by the State. The former is a trivial indication of personal taste – a choice, say, between Elvis Presley, John Lennon and Kurt Cobain. The latter is interesting in the abstract, because places like Choeung Ek and Auschwitz are milestones, however dark. They are also a lesson on morality, better remembered than forgotten because they help us to identify evil, and the trivial and universal aspects of dark tourism are probably best separated by asking how, a question that isn’t normally useful at a grave, but might be the only way to find a lesson in mass atrocity.

By the time I was nine, Apartheid was being dismantled. I remember the first black boy to join my class, but only in retrospect, because I didn’t recognise his arrival as out of the ordinary at the time. When the whole edifice finally went, in 1994, I had a simple, child’s notion of why the long, mixed-race queue outside my neighbourhood voting station was important. A friend and I sold cold drinks up and down the line, carried along by the euphoria. I don’t remember seeing Whites Only signs or passbooks in my narrow suburban world, and if my naivety helps to explain the longevity of Apartheid and systems like it, it is also an indication of how my picture of South Africa was mostly put together in the years after 1994, when it was tempting to think of the country as a blank page. Apartheid’s laws were gone. Its economic underpinnings might have been left largely untouched, but removing them was tricky: the whole country could have collapsed.

I took this blithe, middle-class optimism with me when I visited Johannesburg’s Apartheid Museum in 2008. By then, I had reconstructed the history of Apartheid at high school and deconstructed post-colonialism at university. I thought I knew both well. Claire and I bought our tickets together, but like every visitor we were randomly classified as different races: she as non-white, I as white. The two had separate entrances, which we obediently slipped through. It was only when we were reunited inside that I realised how easily we had obeyed, and how brutality could flow out of the simple placing of signs.

Although the Apartheid Museum had never been a part of the government’s machinery, I learnt something about atrocity by just walking through the door. Choeung Ek on the other hand was the epicentre of Cambodian bloodletting, but it gave visitors no comparable insight into the Khmer Rouge’s reign of terror. It was a monument to violence as much as its victims, without any reminder of how easily people slip into the role of aggressor and aggrieved, and it allowed tourists to think of the genocide as peculiarly Cambodian – as a disease like malaria or typhoid, which didn’t exist at home. It is unlikely to improve: since 2005, the site has been leased to JC Royal for $15,000 a year. The Japanese company makes its profit on entrance fees; in return, it is expected to “develop and renovate the beauty of Choeung Ek killing fields.”

In January 1976, S-21 moved to Takhmau, on the southern outskirts of Phnom Penh…In June, the prison moved again to new premises: the former high school, now known as Tuol Sleng. This site could hold up to fifteen hundred prisoners at a time…By early 1977, Tuol Sleng employed at least 111 warders…These people were beholden to the Centre, not only because of their geographical origins, but also because of their very young age. Eighty-two of the one hundred and eleven warders were aged seventeen to twenty-one…These people were to imprison and kill the vast majority of veteran CPK cadres…Factory workers in Phnom Penh, who knew about the centre’s existence, but not about what went on inside its barbed-wire walls, called it the ‘place of entering, no leaving’. Only half a dozen of the twenty thousand men and women taken there for interrogation survived.

Ben Kiernan, The Pol Pot Regime

It is a promise of death, rasped through cold lips: “Tuol Sleng” – Strychnine Hill – a name matched perfectly to a place. It is also a banality, something tuktuk drivers yell when they slow down to match your stride: “Tuol Sleng! Killing Fields! You go took-tooook!”

Your confrontation with it begins just past the entrance, at a list of rules. “While getting lashes or electrification you must not cry out at all,” reads number six. It is the most brutal, but the rules are uniformly pitiless. There were no suspects here, with hopes of reprieve; it was a place of pain and indignity leading inevitably to death.

You were going to make your way through the former school alone, but already you’re unsteady, unsure of how to process such scrupulous violence. You make your way back to the ticket office instead, to hire a guide. None are available. You wait.

After fifteen minutes a middle-aged woman approaches you. Without introducing herself, she leads you into the first of Tuol Sleng’s classrooms. It is sepia-toned, picturesque. A pool of light collects beneath a rusting bed, with iron rods and tin boxes on top of it – one for beatings, the guide tells you, the other for excrement, but it’s hard to be sure. Her English is heavily accented and she’s speaking quickly, already hurrying you on, into the classroom next door. It is much the same: ochre paint, hues creeping with the damp; checkerboard tiles, amber and off-white spread apart by veins of black; cast iron bed, delicately welded at the head. A black-and-white photograph of the bed has been hung on the wall. It was taken by the Vietnamese in 1979, after they captured Phnom Penh. A corpse is tied to it; blood has collected on the floor in place of light.

Somebody has tucked a frangipani into the foot of the bed. It is still fragrant, and reminds you of the incense beneath the Killing Tree. It was also Animist, also focused on an object: this exact bed, that exact tree, as if they were able to explain the violence, or were somehow to blame. You wonder if the flower has been offered to the spirit of the man in the photograph – if he haunts this room.

After a perfunctory explanation, your guide is striding past a courtyard gallows, into another classroom. The previous two were intimate, like bedrooms, but here doorways have been knocked out of the dividing walls, joining the whole of the ground floor. The next floor up is lined with cells the size of stretchers, intended to hold two, but down here they have all been cleared, to make way for a gallery of pleading faces. Prisoners at Tuol Sleng were meticulously processed: numbers were assigned and confessions extracted, but mutilated bodies were all thrown into the same ghastly pit.

Your guide points out a stool in the corner, with brass attachments. It looks like something used for torture – like the water boards, hooks and pincers also on display – but it was only for identity photographs, to ensure the condemned held their heads perfectly in place. Row after row of the black-and-white results are on display. A few prisoners have already been beaten, and have black eyes or split lips, but most are untouched, and it is only by looks of abject fear or resignation that you can tell if they grasped their fate.

There are photographs of the warders too: smiling, proud, identically dressed. One warder has before-and-after photographs: the 21-year-old warder, a new recruit with a boy’s face, and the 46-year-old man, his chest covered completely by a sak yant – a sacred tattoo. “When I worked at S-21,” reads the caption, “I did not have the motivation, but I had to, otherwise I wouldn’t live. However, no matter which option I chose, I still feared. There was nothing I could do.”

The guide herds you wearily through the gallery, pausing only to explain feature attractions. Occasionally she starts talking before you’ve caught up, leaving you to make sense of her half-sentences. A video is showing upstairs she tells you, as she leads you on. You shouldn’t miss it.

At the bottom of a flight of stairs she stops, making it clear that this is where you will part. She begins her conclusion, pointing to a nearby map. Her family was torn apart she says, and pointing at the corners of Cambodia, she explains: her parents here, her uncle there, her sisters, her brother, over there. Dead – all of them dead. It is too intimate, and you squirm, but her eyes are hard. This is what you paid for, they seem to say: the taste of another person’s misery.