THE MOST EFFICIENT way to strangle a man is with a wire. There's not too much blood, and they can't get their fingers underneath it, so there's no way to pull it off of their neck.

So explains Anwar Congo, the man at the center of the brilliant new documentary The Act of Killing. Congo learned the method from American gangster films, and he had occasion to test it—many, many times—during Indonesia's 1965 military coup, in which hundreds of thousands of "communists" and ethnic Chinese were massacred. Back then, Anwar Congo was the leader of a death squad; today, he's a white-haired gentleman happy to relive his glory days with the help of documentary filmmaker Joshua Oppenheimer.

When critics say that a movie will "challenge audience expectations," they usually mean something along the lines of: The characters are multidimensional. The pretty girl ends up with the fat friend. The good guy doesn't live happily ever after. And when "you won't believe your eyes" is tossed around, it's because some supercomputer spewed out some pretty sweet CGI.

What these phrases don't often mean is that a film will uproot beliefs about human decency so deeply held that you didn't even realize you had them, or that there will be moments when you will find yourself literally unable to trust the truth of what you are seeing.

So it is in The Act of Killing, in which Anwar Congo and his friends set out to film re-creations of their activities during the Indonesian coup, approaching the project with the glee of kids filming a skit. (For more on the conflict, "read a book," as Errol Morris, who produced the film along with Werner Herzog, has told detractors who criticize the film for lack of context.)

Congo dyes his hair black, so he looks more like his younger self. He and his friends create elaborate prosthetics to dummy up scenes of murder and torture; they recruit women and children to stand in as victims in scenes of mass violence. The frankness with which they describe their actions is appalling, their lack of remorse is unsettling, and the campy, low-budget techniques they use to film scenes of rape, torture, and murder is... well, you won't believe your eyes. But while many of the scenes that Congo and his associates film are over-the-top ridiculous, when Congo strolls through the flames of a just-torched village, a cowboy hat resting on the back of his neck, it's nothing but chilling.

"We murdered people and were never punished," says another war criminal in voiceover, as he browses for T-shirts at the mall with his family. Another recounts killing his girlfriend's father; still another reminisces about raping 14-year-old girls. No one is sorry. They've never been punished. As one man puts it, "War crimes are defined by the winner."

Congo is the only one who appears troubled by remorse—who appears to understand, in the slightest, that he might have perpetrated something horrific, and as the film progresses, revisiting his past deeds proves unexpectedly upsetting. But Congo is alone in this, at least among the men Oppenheimer depicts. Most of these men see their actions as justified; because they're still in positions of power, Indonesian society largely validates their position. The most horrific thing about The Act of Killing—the thing reflected with such clarity in the scenes that these men recreate—is that most of them feel nothing at all.