As a child growing up in London, I was blissfully unaware of other worlds less safe and secure than my own. That all changed when I was about 12. Leafing through National Geographic, I started reading a feature about some ancient ruins in Cambodia that looked very beautiful. But what really caught my attention was an article next to it about the country waking up from the nightmare of the Khmer Rouge.

Seeing those images of victims' skulls and mass graves was a defining moment for me. I couldn't believe there were countries where crimes such as this could happen – what really terrified me was finding out that members of the Khmer Rouge still hadn't been brought to justice. I started to read up about the country; how Pol Pot's regime had wanted an agrarian revolution where life would be very simple, which had instead resulted in horror and bloodshed. Nearly 2 million people had been killed outright or died as a result of torture, overwork or starvation in the latter half of the 1970s.

More than half a decade later, still fascinated, I went to art school, but dropped out after a year, realising there was only one thing I wanted to do – to travel to Cambodia to make sense of it myself.

On my second day in the country, I made a beeline for a memorial site at Tuol Sleng prison, an interrogation centre where confessions were forced out of alleged spies and saboteurs. The man in charge, Comrade Duch, had personally overseen the torture and execution of at least 14,000 people. It was Duch's portrait in the prison that fuelled my interest in tracking him down. I thought if there was anyone who could explain how these atrocities had come about, it was him.

Over the next few years, I worked in Bangkok as a photographer, making trips back to Cambodia, always carrying a photo of Duch to show defected Khmer Rouge members. I never believed I'd find him. But in 1999 I made a breakthrough. By chance, on another assignment, I travelled to a nearby Khmer Rouge area that had just opened up. I was wandering around when a small, wiry man in an African Refugee Committee T-shirt came and introduced himself as Hang Pin.

I knew immediately who he really was. It was the same face I'd been carrying around with me for more than a decade. Duch was a little bit greyer, but there was no doubt in my mind. We had a fairly banal conversation – he was interested in my camera, and I tried to appear as nonchalant as I could. This was no ranting, cold-eyed madman; he was garrulous, friendly, disarming. He told me he was a humanitarian aid worker and lay-preacher, converting Cambodians to Christianity. Surreptitiously I took a photograph, but it didn't seem the right time to confront him. I wondered if he might still be a killer.

Later, I returned to the village with Nate Thayer – the last western journalist to have interviewed Pol Pot. We talked to Duch about land mines and his planned church, but he dodged any leading questions about his past. It was only when he asked to see Nate's business card that we realised Duch had suspicions of his own.

"Nic," he said, "I believe your friend has interviewed Pol Pot."

"That's right," I said.

Duch gave a deep sigh: "I believe it is God's will you are here," he said. It was almost as if he'd always expected this day to come.

After that he talked openly about what had happened and said he was very sorry. I'd spent 10 years pursuing a "monster", and found instead a wizened old man who appeared to be contrite and displaying humanity. We showed him confessions from Tuol Sleng prisoners, and he identified his handwriting on them.

A few days later, Duch simply gave himself up to the Cambodian authorities, but it took another decade for him to be tried through a United Nations-backed tribunal. Found guilty of crimes against humanity, he faces 35 years in jail.

My part in this process has been very small; finding him was an accident, and the chain of events since has had nothing to do with me. It wasn't my plan to bring him to justice; I simply wanted to see for myself how this man – a former mathematician and teacher – had become one of the most notorious mass murderers of the 20th century.

• As told to Chris Broughton.

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• This article was amended on 9 September 2010. The original mispelled the contributor's name as 'Nice', which no doubt he is. This has been corrected.