Khmer Rouge torturer-in-chief Duch once taught maths to school children but put his cold, calculating mind to far more devastating use as head of a jail from which few inmates ever came out alive.
The 67-year-old -- whose real name is Kaing Guek Eav -- oversaw the extermination of some 15,000 men, women and children at the Tuol Sleng prison in Cambodia's capital during the communist regime's brutal 1975-1979 rule.
Those who worked under him at the prison testified that Duch was universally feared by the staff.
Most who worked 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 war crimes 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, and he was sentenced to 30 years in prison Monday for war crimes and crimes against humanity.
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 cadre 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 schoolbook 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 only a few dozen prisoners turned into a daily torrent of condemned coming through Tuol Sleng, or S-21, as the regime purged itself of its "enemies".
Ever meticulous, Duch built up a huge archive of photos, confessions and other documents with which prosecutors traced the final horrible months of thousands of inmates' lives.
Following the Khmer Rouge's fall from power, he maintained posts within the communist movement as it battled 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.
After his wife was murdered in 1995, Duch turned to Christianity.
He was arrested after Irish photojournalist 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.
"I told Nic Dunlop, 'Christ brought you to meet me'," Duch told his trial. "I said, 'Before I used to serve human beings, but now I serve God'."