Duch, who oversaw the deaths of 15,000 people as a Khmer Rouge prison chief in the 1970s, portrayed himself Sunday as a victim of circumstances in a documentary screened at Cannes.
Cambodian director Rithy Panh filmed Duch a few weeks before the commander of Tuol Sleng prison, also known as S-21, became the first Khmer Rouge cadre to be tried by a UN-backed tribunal in Phnom Penh.
Panh apologised Sunday for turning down interviews about "Duch: Master of the Forges of Hell", saying a decision was imminent on Duch's appeal of a 30-year jail sentence for war crimes and crimes against humanity.
"I don't have the right to set out my opinions" while the appeal process is under way, said the film-maker, who was a child when his family perished under the Khmer Rouge.
"Duch: Master of the Forges of Hell", which in the style of French documentaries does away with a narrator, sees Duch speaking calmly and frankly from behind a desk carpeted with photos of his victims.
He does not deny his actions, but puts them in the context of the brutality with which the Khmer Rouge, under Pol Pot, imposed its brand of Marxism-Leninism on then-Kampuchea in the late 1970s.
"I tend to regard myself as innocent" and "held hostage" by the regime, said Duch, who today is 68.
"I just belonged to the police... I wanted to go up the ladder just like anyone else," he added.
Speaking in Khmer, but switching to French to quote Karl Marx and the International Declaration of Human Rights, Duch described himself to be "a stoic, not a sadist".
For the Khmer Rouge revolution to succeed, he recalled, it was deemed necessary at the time for detainees at S-21 to be tortured, then interrogated, then "destroyed".
"This is how the machine worked," he said.
Spliced into the film are re-enactments of interrogations at S-21 - a one-time high school that now is a genocide museum - and paintings from Vann Nath, who survived his incarceration there.
Led by "Brother Number One" Pol Pot, who died in 1998, the Khmer Rouge emptied Cambodia's cities in the late 1970s in a bid to create an agrarian utopia, killing up to two million through starvation, overwork and genocide.