The documentary uses retrospective and personal testimony of those who carried out the killings. One death squad leader, Anwar Congo, who killed some 1,000 people, demonstrates in the film his “favorite” way of killing: strangling victims to death with an iron wire to avoid spilling too much blood. He and his co-culprits say they celebrated the killings by singing and dancing.

Half a century has passed and none of them have been held accountable; nor do they have any sense of guilt or words of remorse. The remarks of one of Anwar’s accomplices in the film may explain why: “Crimes are defined by the winners. I’m a winner, so I can make my own definition”.

The perpetrators were actually boastful — when approached by Oppenheimer — and eager to re-create the scenes of killings and reveal their feelings. The film follows that process and documents its consequences.

I was shocked that so many lives were lost in one of the darkest periods in history. I was surprised to find that not enough attention has been paid to a massacre of such scale. And I was infuriated that many years after the crime against humanity, the perpetrators are still at large, proud of what they did. Actually the paramilitary organization that grew out of Anwar’s death squads is still powerful, with its leaders even holding powerful political posts.

The 1965 incident is reminiscent of the Nanjing Massacre in 1937, when invading Japanese troops killed 300,000 Chinese civilians and unarmed soldiers after capturing the then Chinese capital, or the Nazi regime’s attempt to annihilate the Jews during World War II. Sirens are sounded each year in Nanjing to remind people of the tragedy and that dark chapter in history. And most of the Nazi perpetrators have been brought to justice, some of them hunted out from remote corners of the world. It appears surreal and scary to find that people guilty of committing the same crimes are still challenging our conscience by boasting of their evil acts in front of the camera.

“I thought I had wandered into Germany 40 years after the Holocaust to find the Nazis were still in power,” Oppenheimer said of the men who feature in his documentary.

This is a stain on our collective conscience. Crime, if not atoned, is crime committed twice.

The author is a senior writer with China Daily. huangxiangyang@chinadaily.com.cn.