Douch is the former head of the terrible Tuol Sleng prison in Phnom Penh, also known as S-21, where 15,000 people were tortured before being executed between 1975 and 1979.
He will remain as a deeply ambiguous personality who cooperated with justice and cry emotionally at first hearing.
The personality of Kaing Guek Eav, alias Douch, has never been unanimous.
During his first trial, between March and November 2009, this man with grey hair and a powerful gaze endorsed the torture, cruelty as a political method to execute masses.
“I am emotionally and legally responsible,” he admitted.
Converted to Christianity in the 1990s, he asked forgiveness from the few survivors and families of the victims, agreeing to be sentenced to death.
But on the last day of the hearing, he offered a resounding twist, arguing that he was only a servant and not a senior official of the regime and that he therefore not comes under the jurisdiction of the court.
Sentenced to 30 years in prison for crimes against humanity and war crimes in 2010, he appealed.
Then separated from his lawyer, to obtain his release, he was then sentenced to life imprisonment.
He was a key witness in the trial of three Khmer Rouge leaders, the only ones to have been held responsible for the deaths of two million people.
Born on November 17, 1942, in a village in the province of Kompong Thom, north of Phnom Penh, Douch was a professor of mathematics before joining the Khmer Rouge in 1967.
After the fall of the regime in 1979, he continued to belong to the movement and then worked for humanitarian organizations.
After years in hiding, he was found in 1999 by an Irish photographer and arrested.
He also portrayed himself as a prisoner of doctrine, unable to say no.
And refused to take on a political role within the Khmer Rouge regime (1975-1979), taking refuge behind the fear of being shot down to justify his role.