WHEN a documentary tries to focus on evil, when it zeros in on the people who committed unspeakable acts, there's a frustrating diffusion. It somehow never finds the target. The evil is always somewhere else.

There's the testimony of Nazis, for instance, who insist they were only following orders. It was the fault of their superiors. They seem so reasonable, so disquietingly normal. Or the mass killer who speaks with detachment about his (usually his) victims and, quite often, the extenuating circumstances (abused as a child, etc.) that turned him into a killer. Suddenly the humanity of the person, the distancing of time, and the fact that this conversation is taking place in civil circumstances, all combine to pollute the moral clarity we desperately seek.

The same disquieting phenomenon occurs in Rithy Panh's "S21: The Khmer Rouge Killing Machine," a modest but nonetheless devastating documentary about the kind of brutality that was official procedure in Cambodia a generation ago.

In its conquest and occupation of Cambodia, the Khmer Rouge army slaughtered approximately 25 percent of the Cambodian people between 1975 and 1979. Its methods of interrogation, as we learn, were not only cruel but absurd. Prisoners were beaten and abused until they revealed the names of enemies of the new state: the Communist Party of Democratic Kampuchea. Unable to think of anyone, the victims would simply name the people they knew. Those named people were then hauled in for systematic cruelty and inevitable death. All were killed, no matter what they said.

As the movie shows, one of the central points for this inhumane activity was in Phnom Penh at the S21 "security bureau," where 17,000 detainees were killed. (A total of 1.7 million Cambodians perished.) Barely more than a dozen survived; and only three, it seems, have survived to give their testimony for this film.

It's a white-knuckle experience to listen to these former prisoners, to watch them break down emotionally as they visit this place (now it's the Tuol Sleng museum) after many years. One survivor, an artist, relates how he was allowed to live because he could render flattering portraits of the guards.

Director Panh, who managed to escape from Cambodia in 1979 and now lives in France, also talks to young men who were the guards of this hell. They were also executioners. They dug graves, killed people and buried them. Back then they were teenagers, instructed to beat, torture and kill. Now, they are still relatively young. But they cannot permit themselves to take the blame. Had they not complied with orders, they say, they would have been executed by the Angkar, or Organization, as well. And yet, in the most surrealistic of moments, one of those guards reenacts -- with a sickening authority no professional actor could achieve -- his routine of feeding, harassing and yelling at the prisoners. He does it with a fluidity and a joy of performance. It's harrowing and enlightening. And somehow the evil floats away.

Leaders such as Khieu Samphan, Nuon Chea (the Khmer Rouge's chief ideologue), Ta Mok and Kaing Khek Iev, all await trial for genocide. They are not in this film. We see only their work. And their underlings. And once again, evil remains elusive.

S21: THE KHMER ROUGE KILLING MACHINE (Unrated, 101 minutes) -- Contains harrowing anecdotes of a truly disturbing nature. In Khmer with subtitles. At the Avalon Theatre.