If those murders are less well known than others (like the Khmer Rouge slaughter in Cambodia a decade later), it is partly because Indonesia has not undergone the kind of public reckoning that often follows such catastrophes. “History is written by the winners,” muses one of Mr. Oppenheimer’s subjects, who led a right-wing death squad in North Sumatra. “And we are the winners.”

He and his former colleagues, who still keep in touch and occasionally socialize with one another, lead comfortable, even privileged lives. Some serve in provincial or national governments. They speak at rallies of Pancasila Youth, a nationalist paramilitary group still apparently popular — and also still feared — for its role in wiping out suspected Communists almost 50 years ago.

Perhaps the most startling thing about the killers in “The Act of Killing” is that they seem to have no interest in denying, excusing or minimizing their crimes. On the contrary, they are candid, even boastful about what they have done, and eager to share their recollections of torture and murder. “Never forget” is traditionally the slogan of victims fending off revisionism, indifference and the passage of time, but in this case the killers themselves seem most interested in keeping the memory alive.