Inside the mind of a torturer.
The 67-year-old - whose real name is Kaing Guek Eav oversaw the extermination of 15,000 men, women and children at the Tuol Sleng prison in Cambodia's capital during the communist regime's 1975-1979 rule. He has now been sentenced to 35 years in prison for war crimes and crimes against humanity the first verdict involving a senior member of the "killing fields" regime that devastated a generation of Cambodians.
Those who worked under him at the prison testified that Duch was universally feared by the staff. Most of them there were uneducated teenage boys, whom Duch said could be easily indoctrinated because they were "like a blank piece of paper".
'Comrade' Duch begged for forgiveness at Cambodia's UN-backed court for crimes committed under his command at the jail, where prisoners were tortured into denouncing themselves and others as foreign spies.
But victims questioned whether his remorse was genuine after Duch asked to be acquitted in his closing remarks in November.
As a staunch communist, then a born-again Christian and finally remorseful defendant, Duch seemed to always strive to please those above him, making his request to be released all the more surprising. "He is meticulous, conscientious, control-oriented, attentive to detail and seeks recognition from his superiors," according to a psychological examination released by the UN-backed court.
Born in 1942 in central Cambodia, Duch is remembered as a sincere teacher devoted to helping the poor before he became a Khmer Rouge henchmen in 1970.
The decision to join the communist guerrilla movement was influenced by one of his high-school instructors, who also enlisted but would later be executed at Tuol Sleng as a suspected traitor.
"I joined the revolution in order to transform society, to oppose the government, to oppose torture," Duch said during his trial.
"I sacrificed everything for the revolution, sincerely and absolutely."
Inside the rebel-controlled zones, he chose Duch as his revolutionary name because it was used by a model student in a school book from his youth. He then oversaw a series of jungle prisons before being made head of Tuol Sleng after the regime seized the capital in 1975.
What began as 'punishing' just a few dozen prisoners turned into a daily killing ritual of the condemned coming through Tuol Sleng, or S-21, as the regime sought to purge itself of its 'enemies'.
Ever-meticulous, Duch built up a huge archive of photos, confessions and other documents with which prosecutors used to trace the final horrible months of thousands of inmates' lives.
Following the Khmer Rouge's fall from power, he maintained his posts within the communist movement as it battled with Vietnam-backed troops. He also reportedly worked in the 1980s for Radio China and later taught English and maths in at least one refugee camp. He was arrested after Irish photo journalist Nic Dunlop uncovered him working for a Christian aid agency in western Cambodia under a false name.
Before that, many had long assumed he was dead following his disappearance after Vietnamese troops ousted the Khmer Rouge in 1979.
Duch said during his trail: "I told Nic Dunlop, 'Christ brought you to meet me'. I said: 'Before I used to serve human beings, but now I serve God'."
Cambodia's journey from horror to justice:
The communist Khmer Rouge, led by 'Brother Number One' Pol Pot, seized control of Cambodia in April 1975 and began dismantling society in its drive to transform the country into an agrarian utopia.
The regime abolished religion, schools and currency, and exiled millions of people onto farms.
Up to two million people died of starvation and overwork or were executed from 1975 to 1979. The horrors of life under the Khmer Rouge were portrayed in the Hollywood film 'The Killing Fields'.
WHAT HAPPENED NEXT:
The Khmer Rouge was driven from power in 1979 by Vietnamese troops and former regime members who defected, including Hun Sen, now Cambodia's prime minister. Under him, the Cambodian government fought the Khmer Rouge until the movement collapsed in the mid-1990s.
Cambodia and the UN signed an agreement in 2003 which brought the tribunal into being. Known as the Extraordinary Chambers of the Courts of Cambodia (ECCC), it is a hybrid court combining international law with Cambodia's judiciary.
Its mandate is to prosecute "those most responsible" for crimes between 1975 and 1979.
WHO FACES TRIAL:
Leader Pol Pot died under house arrest in 1998 before he could stand trial.
After Duch, the court may try 'Brother Number Two' Nuon Chea, former head of state Khieu Samphan, foreign minister Ieng Sary and social affairs minister Ieng Thirith, on charges of genocide and war crimes.
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