Although he sent thousands to their deaths, former S-21 prison chief Kaing Guek Eav kept his own hands clean.

On his 12th and final day of testimony before the Khmer Rouge tribunal yesterday, Eav, better known as Duch, said despite overseeing the interrogation and execution of more than 12,000 people who were rotated through the prison, he took steps to ensure he didn’t personally end a life.

Nuon Chea defence lawyer asked Duch about an incident he mentioned during his testimony, in which he claimed Chea gave him potentially poisonous capsules to feed to prisoners, but Duch replaced the powder with paracetamol.

Koppe said he was puzzled by Duch’s account of swapping the powder in an apparent – and temporary – salvation for a handful of prisoners who were already marked for death.

“You were responsible for executing all these prisoners …What made you want to save these people?” Koppe asked. “I did not allow my hand to kill prisoners,” Duch said.

“The medicine was given by Uncle Nuon himself, so if I did not want to kill prisoners by myself, I needed to change the contents.”

Duch also expounded on post-1979 “fabrications” at the prison – including a large immersion basin for water torture, and a chalkboard on which S-21 rules were inscribed.

Duch claimed these features were added by the Vietnamese after S-21 staff had fled the site, which now stands as the Tuol Sleng Genocide Museum. “I denied that particular basin existed, and I made my protest,” Duch said.

Drawing on a list of 15,000 names of S-21 prisoners, Koppe noted that less than 5 per cent were listed as the “wife of” another prisoner, but Duch could not put a figure to the number of family members arrested for the sins of their kin.