I have spent time with mass killers, warlords and death squad leaders as a reporter in Latin America, the Middle East, Africa and the Balkans. Some are psychopaths who relish acts of sadism, torture and murder. But others, maybe most, see killing as a job, a profession, good for their careers and status. They enjoy playing God. They revel in the hypermasculine world of force where theft and rape are perks. They proudly refine the techniques of murder to snuff out one life after another, largely numb to the terror and cruelty they inflict. And, when they are not killing, they can sometimes be disarmingly charming and gracious. Some are decent fathers and sentimental with their wives and mistresses. Some dote on their pets.

It is not the demonized, easily digestible caricature of a mass murderer that most disturbs us. It is the human being.

Joshua Oppenheimer's documentary "The Act of Killing," which took eight years to make, is an important exploration of the complex psychology of mass murderers. The film has the profundity of Gitta Sereny's book "Into That Darkness: An Examination of Conscience," for which she carried out extensive interviews with Franz Stangl, the commandant of Treblinka, one of the Nazi extermination camps. Oppenheimer, too, presents candid confessions, interviewing some of the most ruthless murderers in Indonesia. One of these is responsible for perhaps 1,000 killings, a man named Anwar Congo, who was a death squad leader in Medan, the capital of the Indonesian province North Sumatra.

The documentary also shows the killers performing bizarre re-enactments of murders.

Indonesia's military, with U.S. support, launched in 1965 a year-long campaign to ostensibly exterminate communist leaders, functionaries, party members and sympathizers in that country. By its end, the bloodbath -- much of it carried out by rogue death squads and paramilitary gangs -- had decimated the labor union movement along with the intellectual and artistic class, opposition parties, university student leaders, journalists, ethnic Chinese and many who just happened to be in the wrong spot at the wrong time. By some estimates, more than a million people were slaughtered. Many of the bodies were dumped into rivers, hastily buried or left on roadsides.

This campaign of mass murder is still mythologized in Indonesia as an epic battle against the forces of evil and barbarity, much as U.S. popular culture for many decades mythologized our genocide of Native Americans and held up our own killers, gunmen, outlaws and murderous cavalry units of the Old West as heroes. The onetime killers in the Indonesian war against communism are cheered at rallies today as having saved the country. They are interviewed on television about the "heroic" battles they fought five decades ago. The 3-million-strong Pancasila Youth -- Indonesia's equivalent of the Brown Shirts or the Hitler Youth -- in 1965 joined in the genocidal mayhem, and now its members, like the death squad leaders, are lionized as pillars of the nation. It is as if the Nazis had won World War II. It is as if Stangl, instead of dying in the Duesseldorf remand prison as a convicted war criminal, came to be a venerated elder statesman as has Henry Kissinger.

There is a scene in the Oppenheimer film where Congo -- who parades across the screen like a prima donna, his outsized vanity and love of fine clothing on display -- is interviewed on "Special Dialogue," a program of a state-owned television station with national coverage. I have substituted the word "Jew" for "communist" to put the moral bankruptcy of the Indonesian regime into a cultural context better understood by Americans.

"We had to kill them," Congo, wearing a black cowboy hat adorned with a gold sheriff's star, tells the female host.

"And was your method of killing inspired by gangster films?" she asks.

"Sometimes!" Congo says. "It's like. ... "

"Amazing!" she says. "He was inspired by films!"

The audience, mostly made up of members of the Pancasila Youth in their distinctive orange and black shirts, applauds. At the start of the show, Ibrahim Sinik, a leader of the paramilitary group, lauded the Pancasila Youth as having been "at the core of the extermination."

"Each genre had its own method," Congo says. "Like in Mafia movies, they strangle the guy in the car, and dump the body. So we did that too."

"Which means Anwar and his friends developed a new, more efficient system for exterminating Jews," the woman says enthusiastically, "a system more humane, less sadistic, and without excessive force."

"Young people must remember their history," Ali Usman, a Pancasila Youth leader, interjects. "The future musn't forget the past! What's more, God must be against Jews."

"Yes," the talk show host says. "God hates Jews!"

There is more applause.

Oppenheimer, in the film's strangest but most psychologically astute device, persuades the killers to re-enact some of the mass murders they carried out. They don costumes -- they fancy themselves to be the stars of their own life movies -- and what comes out in the costumed scenes of torture and killing is the vast disconnect between the image they have of themselves, much of it inspired by Hollywood gangster films, and the tawdry, savage and appalling crimes they committed. 

These scenes include one of the old killers named Herman Koto -- Koto and the other murderers refer approvingly to themselves as gangsters -- done up to look like the drag queen Divine. And in these moments Oppenheimer captures the playfulness, the black humor and the comradeship that create bonds among killers. The killers stage a scene at the end of the film in which actors playing their murdered victims hang a medal around the neck of Congo -- who is dressed in a long, black robe and standing in front of a waterfall -- and thank him for saving the country and "killing me and sending me to heaven." This bizarre fantasy's background music, specified by Congo, is the theme from the movie "Born Free."

These same human bonds, along with the same schizophrenic self-delusion, can be glimpsed in photographs of off-duty Nazis in the book "Nein, Onkel: Snapshots From Another Front 1938-1945," or in the photographs of off-duty SS camp guards at Auschwitz. One of the pictures in the Auschwitz album shows the SS leadership, including the commandant of Auschwitz, Rudolf Hoess, and Dr. Joseph Mengele, who carried out inhuman medical experiments on children, in a raucous "sing-along" on a wooden bridge with an accordion player at Solahutte, an SS resort about 20 miles south of Auschwitz on the Sola River. Mothers and children not far away were being gassed to death, some of the 1 million people murdered at Auschwitz. And it is this disquieting moral fragmentation, this ability to commit mass murder and yet to see oneself as a normal, caring human being, that Oppenheimer astutely captures. The bifurcation between work and life -- a bifurcation that many in the U.S. military, today's fossil fuel or health insurance industry or Wall Street firms such as Goldman Sachs also must make -- allows human beings who exploit, destroy and kill other human beings to blot out much of their daily existence. 

"I killed every Chinese I saw," Congo remembers as he tours the Chinese area of Medan in a car. "I stabbed them all! I don't remember how many, but it was dozens of Chinese. If I met them, I stabbed them. All the way to Asia Street, where I met my girlfriend's dad. Remember, I had two motives: crush the Chinese and crush my girlfriend's father, so I stabbed him, too! Because he was Chinese too! He fell into a ditch. I hit him with a brick. He sank."

"Killing is the worst crime you can do," says one of Congo's former associates. "So the key is to find a way not to feel guilty. It's all about finding the right excuse. For example, if I'm asked to kill somebody, if the compensation is right, then of course I'll do it, and from one perspective it's not wrong. That's the perspective we must make ourselves believe. After all, morality is relative."

Congo patiently explains to Oppenheimer his technique of garroting his victims with a piece of wood, a pole and wire, a technique he adopted to avoid the mess of excessive bleeding.

"There's probably many ghosts here, because many people were killed here," he tells Oppenheimer as they stand on a rooftop at one of his former murder spots. "They died unnatural deaths -- unnatural deaths. They arrived here perfectly healthy. When they got here they were beaten up..."

Congo crouches and puts his hands over his white, curly hair to imitate the last moment of his victims.

"... And died," he goes on. "Dragged around. Dumped. In the early days we beat them to death. But when we did that blood spurted everywhere. It smelled awful. To avoid having blood everywhere, I used this system."

He holds a piece of wood, about two feet long, and long wire.

"Can I demonstrate it?" he asks.

He secures the wire by wrapping one end of it around a mounted pole. A friend, whose hands are behind his back, sits on the floor near the pole. Congo loops the wire around his friend's throat. Standing several feet away, Congo pulls lightly on the wood, attached to the other end of the wire, to show how the victim was killed.

"I've tried to forget all this by listening to good music," Congo says when he finishes his demonstration. "Dancing. I can be happy. A little alcohol. A little marijuana. A little -- what do you call it? -- Ecstasy. Once I'd get drunk, I'd "fly' and feel happy. Cha cha."

He begins to dance on the rooftop in his white pants and white shoes.

"He's a happy man," his friend says.

"We shoved wood in their anus until they died," Adi Zulkadry, a death squad leader, says later in the film as he is shown shopping in a mall in the capital Jakarta with his wife and daughter. "We crushed their necks with wood. We hung them. We strangled them with wire. We cut off their heads. We ran them over with cars. We were allowed to do it. And the proof is, we murdered people and were never punished. The people we killed, there's nothing to be done about it. They have to accept it. Maybe I'm just trying to make myself feel better, but it works. I've never felt guilty, never been depressed, never had nightmares."

In one scene a film crew member, his raw emotion broken by nervous laughter, says his family was on the receiving end of the terror.

" ... If you want a true story, I have one," the crew member volunteers. "Tell us," Congo responds, "because everything in this film should be true."

"Well, there was a grocery store owner," the man begins hesitantly. "He was the only Chinese person in the region. To be honest, he was my stepfather. But even though he was my stepfather, I lived with him since I was a baby. At 3 a.m., someone knocked on our door. They called my dad. Mom said, 'It's dangerous! Don't go out.' But he went out. We heard him shout, 'Help!' And then, silence. They took him away. We couldn't sleep until morning."

"How old were you?" he is asked.

"Eleven or 12," he answers. "I remember it well. And it's impossible to forget something like this. We found his corpse under an oil drum. The drum was cut in half and the body was under it, like this," he says as he doubles over a piece of paper to illustrate. "His head and feet were covered by sacks. But one foot poked out like this." The crew member raises one foot off the ground. "So the same morning, nobody dared help us," he says.

"We buried him like a goat next to the main road," he says with a forced smile as if the burial story should be amusing. "Just me and my grandfather, dragging the body, digging the grave. No one helped us. I was so young. Then, all the communist families were exiled. We were dumped in a shantytown at the edge of the jungle. That's why, to be honest, I've never been to school. I had to teach myself to read and write."

"Why should I hide this from you?" he says to the former death squad leaders, who listen with wry smiles. "This way, we can know each other better. Right? I promise I'm not trying to undermine what we've done. This isn't a criticism. It's only input for the film. I promise, I'm not criticizing you."

Congo and the other killers dismiss his story as inappropriate for the film because, as Herman Koto tells the crew member, "everything's already been planned."

"We can't include every story or the film will never end," another death squad veteran says.

"And your story is too complicated," Congo adds. "It would take days to shoot."

The killers in the film no longer wield the power that comes with indiscriminate terror, although they periodically wander through local markets to extort money from shopkeepers, a practice Oppenheimer captures on film.

When they carry out murder re-enactments, however, it triggers memories of a time when they were more than petty criminals, when they had license to do anything they wanted to anyone they chose in the name of the war against communism. 

"If they're pretty, I'd rape them all, especially back then when we were the law," one of the killers remembers. "f*ck 'em! f*ck the sh*t out of everyone I meet."

"Especially if you get one who's only 14 years old," he adds after he and some other death squad veterans pantomime molesting a girl and holding a knife to her throat. "Delicious! I'd say, it's gonna be hell for you but heaven on earth for me."

There are moments, usually years after their crimes, when even the most savage killers have brief flashes of self-recognition, although they usually do not reflect upon or examine these revelations. They are often, however, haunted by specific moments of murder. Oppenheimer closes his film with a re-enactment scene where Congo begins by placidly describing the murders he committed at that spot and ends by retching and vomiting.

"I remember I said, 'Get out of the car,'" Congo says of one killing. "He asked, 'Where are you taking me?' Soon, he refused to keep walking, so I kicked him as hard as I could in the stomach. I saw Roshiman bringing me a machete. Spontaneously, I walked over to him and cut his head off. My friends didn't want to look. They ran back to the car. And I heard this sound. His body had fallen down. And the eyes in his head were still. ..."

He trails off.

"On the way home," he finishes, "I kept wondering, why didn't I close his eyes? And that is the source of all my nightmares. I'm always gazed at by those eyes that didn't close."