ANLONG SAN, Cambodia — “We were victims, too,” said Him Huy, the head of the guard detail at the Tuol Sleng torture house, who took part in the executions of thousands of people at a Khmer Rouge killing field.

As the prisoners knelt at the edges of mass graves with their hands tied behind them, executioners swung iron bars at the backs of their heads, twice if necessary, before they toppled forward into the pits.

“I had no choice,” Mr. Him Huy, 53, said. “If I hadn’t killed them, I would have been killed myself.”

Two of Mr. Him Huy’s co-workers at Tuol Sleng, quoted by the historian David Chandler in his book on the prison, “Voices From S-21,” remembered him as “a seasoned killer, an important figure at the prison and a key participant in the execution process.”