More horror arrives in the mail.

Clive Burrage wrote to me about his brother-in-law, Harry Leeks, pilot of an RAF Mitchell bomber in the last two years of the Second World War, based in Cologne in the first months of the occupation of Germany. "He went to get some furniture for his room at the base," Clive told me when I called him in Cheshire. "And he found these photographs in one of the drawers." The pictures slid out of Clive's Do Not Bend packet. Hitler walking in what seemed to be a Warsaw street, presumably in 1939. Goering and Hitler arriving at an airbase. And then a bleak, cold street – soldiers and civilians in overcoats, part of a shop name in what appeared to be Polish – with five bodies hanging by rope from a first-floor balcony.

Their hands are tied behind their backs, their heads askew. And just to the left, is a German soldier taking a photograph of the corpses. I have never seen this picture; it appears to be a "souvenir" taken by a soldier who presumably sent it home to his family – who owned the furniture in which Harry Leeks found it four or five years later.

This picture tells a story, Clive Burrage wrote. "Man's inhumanity to man." A cliche, I know, though first used in this form by Robert Burns. But Clive's letter arrived with another from reader TJ Forshaw who – like Clive – had been moved by the photograph in this column recently of the Nazi Einsatzgruppen officer shooting Jews at Ivangorod in the Ukraine in 1942. "Is it possible under certain circumstances," he asked, "for most people, including the writer of this letter, to become casual killers and on a mass scale? And if the answer is yes they can, why can they then, it seems, sleep soundly in their beds at night without dreaming of committing suicide?"

Quite by chance, in the very same mail packet, came a note from Wies de Graeve of the Flemish Peace Institute, who three years ago invited me to give the Armistice Memorial lecture at Ypres. This year's lecture was given by Lakhdar Brahimi, the Algerian statesman who brought about a ceasefire in Lebanon's 15-year civil war. Brahimi, an old acquaintance of mine, an honourable man, raised the very same question at Ypres as Clive and Mr Forshaw did in their letters: "... the Lebanese are known to everyone, and to themselves, as highly sophisticated, refined, gifted for business as well as for the arts, peaceful and fun-loving. From the early 1970s, however, they surprised the world and themselves and became fierce fighters against each other and against the Israeli invader."

Fighting invaders is one thing, of course, fighting a civil war another. "I have found no ready-made, fully-satisfying answers to these and many other questions," Brahimi said, "neither in the books of academia nor in the tool box of the practitioner. Perhaps the truth is simpler: and that is that we, the members of the human race, are pretty much the same: that, individually or collectively, as the case may be, we are, each and all, capable of both the best and the worst. Circumstances make us one on a given day, and the other on the next."

I'm not satisfied with this explanation. British soldiers, American soldiers, "Allied" soldiers, have done dreadful deeds – in the Second World War, Korea, Malaya, Dutch-ruled Indonesia, Algeria and yes, Afghanistan and Iraq. And their cruelty was part of a culture of impunity and colonialism and government-inspired racism – it did not emerge from the fraudulent "few bad apples" nonsense parroted by George W Bush and Lord Blair of Isfahan. But there was something especially terrible about the Nazis; they belonged to a regime that was irretrievably evil, a society in which not a single element could be identified as anything but bad. Adolf Eichmann's Israeli prosecuting counsel, Avner Less, believed that Eichmann could not exist in a democracy, only in a dictatorship.

But even this explanation is not good enough. Surely the experience of the past decade is that we can encourage, empower and facilitate others to perform awful deeds without taking moral responsibility. No, not on a Nazi scale. But rendition, secret black prisons, mass torture, executions by our allies (in Afghanistan, for example, suffocation to death, electrodes on the genitals in Morocco, sustained torture in Damascus on CIA instructions or in Libya with British collusion). We may no longer be capable of doing these things ourselves. International law – or what is left of it after the criminality of Bush and Blair – still prevents us turning into Nazis. But it doesn't stop us from turning others into Nazis. And I fear that we do sleep soundly.

After all, it's one thing to hold out the hand of human friendship to Muslims – as Barack Obama did in Cairo two years ago – and to opponents of dictatorship; but quite another to supply the dictators with weapons. In Cairo this month, I was shown the cartridge cases of the latest gas grenades to be fired by police thugs at protesters around Tahrir Square – manufactured in 2003, thus long expired – blinding some and killing others, and on the side of each is written: "3231 37/38 MM RIOT CS SMOKE PROJECTILE RANGE 150 YDS (136 METER), COMBINED TACTICAL SYSTEMS 388, KINSMAN ROAD, JAMESTOWN PA (Pennsylvania) 16134 DO NOT FIRE DIRECTLY AT PERSON(S) SERIOUS INJURY OR DEATH MAY RESULT. MADE IN USA.'