Nic Dunlop: When I found Duch living in the jungle I never believed he would answer for his crimes

When I look back to 1999, when I found Duch living in the jungle, I remember thinking how utterly bizarre that he should, in the end, walk up to me. And yet I remember thinking "why shouldn't he?" It wasn't so strange given that very few senior Khmer Rouge members were on any sort of wanted list. The entire issue of justice was a new concept among governments as justice had been sidelined during the Cold War. It was only in the late 90s that putting the Khmer Rouge on trial became a serious consideration.

Ideally, at the time, I hoped there would be a trial and that Duch would be placed in a dock, not only to account for his crimes, but so that he could explain something of the Khmer Rouge horror and how the killing had reached such dizzying proportions; someone with authority within the Khmer Rouge and with daily experience of the killing so that we could not only learn something of the Khmer Rouge but also about ourselves. But I also knew something of the world of "realpolitik". I was realistic about the prospect that any of these people would ever face trial.

Setting up the UN tribunal was quite a challenge. Many Khmer Rouge were old, infirm or dead and the crimes had been committed 20 years before when I met Duch. The trial has cost perhaps as much as $100m (£65m). Compared with other tribunals of this nature, it's not that much. And anyway, how can you put a price on something like this? Plenty of people have voiced complaints, many of them valid, but I think it's impossible to dismiss this process out of hand.

It is hard to say what impact sentencing will have for Cambodians. One thing that is clear is that the trial has generated interest for a new generation in their history that didn't exist before. Before, most young Cambodians knew very little about what happened because the Khmer Rouge were deeply secretive.

That is changing as organisations have reached out to explain the court's business and the Khmer Rouge history. Talking about the Khmer Rouge and learning about it has to be welcomed because to reach a critical understanding of the past is perhaps the closest there is toward preventing this kind of thing from happening again.

There has been much talk of the idea of "pure evil". I'm not convinced that there is such a thing. Duch was committed to carrying out this brutal work because he believed this was the best way to conduct a revolution. In his world view, symptomatic of the times in which he grew up, the Cold War was like the war on terror is to so many now: made up of polar opposites of black and white, right and wrong, "with us or against us" – where the word is divided neatly between two competing narratives with nothing in between. This was a crucial factor in his development and central in creating the frame-work within which the horrors of the Khmer Rouge occurred.

The trial also became an examination of how human beings behave in times of great extremity. At the time, Duch knew that if he refused to carry out his orders that he and his entire family would most likely have been killed. This was the key argument in his defence. The trial forced outsiders to reflect what the individual would do under such a regime. As a result, it becomes impossible to dismiss Duch as a "monster". He's too complex for that.

Duch's trial has forced us to confront the uncomfortable question of what would we have done in his shoes. During the course of the trial Duch wept. But at times he also showed contempt for his victims. For me, watching Duch's performance in court, I've come to believe that we are neither one thing nor another; neither good nor evil. We are both, and everything in between, all at once.

He was not a lone psychopath working against society as a whole; he believed in the revolution. You have to judge him in the context of what was happening around him. To simply label people like the Khmer Rouge as fanatical killers, or today to view the Taliban as "terrorists" or to talk about al-Qa'ida as "enemies", is both intellectually lazy and morally irresponsible. We have to understand what creates movements like this to enable men like Duch to take position and commit these kinds of crimes. Understanding is vital if we're serious about preventing these crimes from being committed in the future.

Nic Dunlop is a photojournalist and author of 'The Lost Executioner'