S21: The Khmer Rouge Killing Machine features the astonishing sight of former torturers from Pol Pot's regime, and a couple of former victims as well, all talking together in the now-empty, sun-drenched rooms of S21, the notorious Cambodian prison where 17,000 people were interrogated and killed. One of the guards describes a particularly ghoulish form of torture in which the prisoners would get an IV stuck in each arm so that their blood could be drained away. ''They breathed like crickets,'' he recalls. ''Their eyes bulged.'' But then he makes a startling confession: Since it was understood that every last one of the prisoners was marked for death, the interrogations served no real purpose -- none, that is, except for offering the torturers an excuse to indulge their darkest impulses. For a moment, you glimpse the soul of a genocidal regime: atrocity committed for an ideological ''reason,'' when, in fact, the atrocity was the reason.
Like all organized mass murder, the staggering terror of the Cambodian genocide can be evoked with numbers. In 1970, the country had a population of 7.7 million; by the time that Pol Pot, the Maoist dictator who seized power in 1975 out of the chaos of Vietnam, was finished ''reeducating'' and slaughtering his people, 1.7 million of them were dead. Yet ''S21,'' unlike many documentaries about the Nazi era, isn't a sickening panorama of brutality. Shot on video, it's quiet and intimate -- a series of dialogues in which half a dozen former guards, in tones of spooky detachment, discuss the things that they did and even act them out within the chambers and corridors of the prison.
At a moment when issues of torture loom so large in our world, ''S21'' testifies to the chilling ease with which ordinary men can cross the line into homicidal sadism. Yet the film, I have to say, sheds little to no light on the psychology of that transformation. What was going through the minds of the Khmer Rouge thugs as they tortured and killed their countrymen? It may be part of the design of human barbarity that such evil remains opaque.