The Khmer Rouge's reign of terror lasted from April 17, 1975, when Pol Pot's forces marched into the capital of Phnom Penh, to Jan. 7, 1979, when Vietnamese troops retook the city. Some 1.7 million Cambodians perished during those years. One quarter or more of the country's population died—murdered by Pol Pot's fanatic followers, starved to death or killed by disease. Rithy Panh is one of the survivors. "The Elimination" is his memoir of those terrible years.

Mr. Panh was 13 years old when the Khmer Rouge ordered his educated, middle-class family to evacuate Phnom Penh for the countryside. The evacuation of Cambodian cities was a first step in the Khmer Rouge's overall plan to create a utopian agrarian society and exterminate what it dubbed the "new people." These were business owners, landlords, professionals, teachers, students—anyone who wasn't a peasant or a laborer and therefore part of the protected class of "old people." When the boy finally escaped, after the Vietnamese invasion, he went first to a refugee camp in Thailand and then on to France, where he became a documentary filmmaker and now lives. "The Elimination," written with the assistance of the novelist Christophe Bataille, is translated from the French by John Cullen.

Within six months of being forced to leave their home in 1975, most of Mr. Panh's family was dead. His father stopped eating as a moral protest against the Khmer Rouge's destruction of his beloved country. A brother headed back to Phnom Penh and was never seen again. A brother-in-law who was a physician was executed at the side of the road, his sole crime being that he was a member of the educated classes. His mother, before she too succumbed, instructed her son to stay alive: "You must keep walking in life, Rithy. Whatever happens you have to keep walking." Through a combination of luck, guile and the kindness of strangers, Mr. Panh managed to do as she had ordered.

People were even stripped of their names as a way to dehumanize them. Mr. Panh became "Mr. Bald," a reference to his head, which had been shaved to get rid of lice. Remembering the Khmer Rouge takeover on April 17, 1975, Mr. Panh writes: "From that day on, I, Rithy Panh, thirteen years old, have no more history, no more family, no more emotions, no more thoughts, no more unconscious. Was there a name? Was there an individual? There's nothing anymore."

Mr. Panh recounts stories of ordinary people who took risks to help him. He once made the mistake of saying a word in French. A Khmer official overheard him speaking the banned language, which could have doomed him as a member of the privileged class, but chose not to report him. Mr. Panh describes such deeds as the "banality of good," reversing Hannah Arendt's famous phrase about the evil of the Nazi killer Adolf Eichmann.

Intermingled with Mr. Panh's personal memoir is a second story—his later confrontation with the mass murderer Kaing Guek Eav, better known as "Comrade Duch." During the Khmer Rouge years, Duch was the commandant of the notorious Tuol Sleng prison, or S-21, in Phnom Penh, where he directed the torture and execution of 14,000 fellow Cambodians.

Mr. Panh interviewed Duch in 2010, when Duch became the first Khmer Rouge leader to be tried and convicted by a United Nations-authorized war-crimes tribunal. Mr. Panh writes that he was not looking for "objective truth" in his conversations with the former leader while he was awaiting trial. He just wanted Duch to "talk and explain himself." Duch did so, as Mr. Panh says, in cultivated Khmer, quoting French poetry and always with a smile on his face. In Duch's account, his victims were not real men and women. "S-21 was the end of the line," Duch told Mr. Panh. "People who got sent there were already corpses. Human or animal? That's another subject."

At Duch's orders—as he himself recorded in notebooks he kept—Cambodians were tortured in horrific ways and made to confess to ideological crimes they often had not committed. Duch was a master of psychological cruelty too. After a prisoner had confessed, Duch would tell him that he was going to be released. The prisoner would be escorted to a truck, but instead of going home, he would be taken to a killing field, where he would be executed, his body thrown into a mass grave.

"The Elimination" is a searing, firsthand account of the Cambodian genocide and as such an important contribution to the history of those years. It is also an examination of the nature of evil as told from the perspectives of a victim and a perpetrator. But it is a difficult book. Perhaps reflecting Mr. Panh's outlook as a filmmaker, it unfolds in a cinematic style. There are no designated chapters; the scenes cut back and forth between Mr. Panh's childhood, his interviews with Duch, and the writer's emotional and sometimes esoteric musings. The narrative is thus not always easy to follow. The reader who is new to the history of Cambodia under the Khmer Rouge might well find himself bewildered.

In 1979, after he had reached safety in France, Mr. Panh wrote a letter to Kurt Waldheim, the U.N.'s secretary-general. The teenager told Waldheim about his suffering and asked him why no one had helped Cambodia. "I didn't understand why no one had come to our aid," Mr. Panh writes in "The Elimination." "It was unbearable: the suffering, the hunger, the death everywhere. And everybody remained silent. We were alone." He never received a reply.

Ms. Kirkpatrick, a senior fellow at the Hudson Institute, is the author of "Escape From North Korea: The Untold Story of Asia's Underground Railroad."