“The Act of Killing” opens Friday at Sundance Cinemas. Not rated, 1:56, three and a half stars out of four. I’ll be doing a post-show discussion in Sundance’s Overflow Bar following the 6:50 p.m. Tuesday, Aug. 27 screening.

Most documentaries about brutal regimes tell their story from the perspective of the victims, their tales of tragedy finally surfacing to the light decades later after the regime has been toppled. Filmmaker Joshua Oppenheimer didn’t have that luxury while making his film about the coup in Indonesia in 1966, when over a million leftists, college students, union workers and others were murdered by government-sanctioned thugs. That regime is still in power, those thugs are still strutting around free. For victims or their families to speak out now would be a death sentence.

So Oppenheimer turns his cameras on the perpetrators in “The Act of Killing,” and the result is one of the strangest and most haunting documentaries I’ve ever seen. The killers, now grandfatherly types in their 50s and 60s, reminisce fondly about their youth spent torturing and murdering innocents, the violence often inspired by what they saw in American movies. Most don’t feel any remorse, because the official record in Indonesia is that they were heroes, stamping out a Communist threat to the nation. And everyone is too scared to say otherwise. “The winner decides what is ‘war crimes’,” one killer says. “And I’m the winner.”

Drawing off that love of movies, Oppenheimer invites the killers to recreate their crimes for the camera in any manner they wish, using sets, costumes, even stage blood. The “re-enactments” are positively surreal, as we the men dress up as movie gangsters in a noir film, or dance under a waterfall in a bizarre music video, the ghosts of the murdered coming forward to thank the killers and present them with medals. It’s so ridiculous that it’s almost laughable, but the laughs stick in your throat.

It’s madness. By turning murder into performance, “The Act of Killing” dramatizes the perverted hearts of these men, the twisted mental leaps they needed to feel okay, and even proud, of what they’ve done. It’s a collective madness that infects the entire country — at one point, one of the men likens his country to a nation of “soap opera stars,” all playing the part of patriotic, happy citizens, none of them believing it.

The one possible exception is Anwar Congo, a dapper man in mustard-yellow suits who has admitted to have killed hundreds of people. He cautiously reveals that his dreams are haunted by the ghosts of the people he strangled, and as he performs in his films, with extras playing the part of screaming women and children, it starts to cut too close to home. If there is a glimmer of hope in “The Act of Killing,” it is in Congo’s slowly growing realization of the horror of what he’s done.

His unease isn’t enough, not nearly enough, to atone for the suffering he caused. But as he revisits a rooftop where he killed his victims, and is suddenly seized by a bout of uncontrollable retching, we’re grateful for this tiny measure of justice. This is an unforgettable film.