ENEMIES OF THE PEOPLE No, wait. This is the most depressing film of the week.

DON'T ASSUME you fell asleep in history class if you're a bit fuzzy on why the Khmer Rouge killed off nearly a quarter of Cambodia's population in the late 1970s. That's the central question in Rob Lemkin and Thet Sambath's documentary Enemies of the People: Why were so many killed? But what's shocking is that the question has remained unanswered this long.

Thet Sambath is a Cambodian journalist whose father, siblings, and mother were all victimized by a party whose legacy is synonymous with famine, disease, torture, and genocide. His father was stabbed to death, his mother was forced to marry a Khmer Rouge soldier, got pregnant, and died in childbirth; in one scene, Thet Sambath stands on the road where he last saw his brother before he too disappeared and was presumably killed. With British documentarian Lemkin remaining in the background, Thet Sambath guides the film with an astonishing lack of vengeance, documenting the recollections of the foot soldiers whose hands cramped up from slitting throats in the killing fields, as well as of the highest ranking member of the regime still alive, Nuon Chea—AKA Brother Number Two, and Pol Pot's right-hand man.

While it's clear that Thet Sambath is obsessed with the historic value of his work, his personal connection to his interviewees and ability to maintain cool control of his emotions is uncannily bright. For the most part, he and Lemkin eschew the shock techniques common in such films, keeping stock footage to a respectful minimum and displaying less of an interest in scandalizing their audience than in completing the important task at hand.

Thet Sambath spent an entire decade's worth of weekends traveling out to the provinces to earn the trust of, and speak to, former soldiers who now look like nothing more than frail old subsistence farmers. They open up on the old methods of slaughter, but the real poignancy in their hanging faces is of guilt and self-disgust. One of them fears that, according to his Buddhist faith, it will be many, many lifetimes until he is fit to reincarnate as a human being; another remembers developing a taste for human gall bladders.

Enemy's greatest achievements, however, are its interviews with Nuon Chea, wherein—for the first time—a member of the party addresses the deaths on record. It's a cathartic moment (as when, during filming, Nuon Chea is hauled off to detention, where he awaits a United Nations trial for crimes against humanity), yet wholly unsatisfying. No matter how much we might yearn for a reason, a halfway relatable excuse for crimes of this scale, there just isn't one. There never is.