The Khmer Rouge erupted from the Cambodian jungle in 1975 and immediately instigated a four-year reign of terror before being routed by the Vietnamese. Within months, they emptied the cities, intent on reducing the population to two classes: workers and peasants. The "new people," an absurdly hopeful term, were executed. These were teachers and students, intellectuals, the middle class, civil servants, landowners, anyone with an education. Ironically, the leaders of the Khmer Rouge were themselves educated men of the upper middle class.

Children were shot and babies dashed against tree trunks. In four years 1.7 million people died. The cadres sought to control every aspect of private life. Only black clothes were allowed, eyeglasses forbidden, love outlawed; all unions were forced marriages. The red revolution was so pure it had no use for human nature. In "The Elimination," Rithy Panh asks, in the tone of contained fury that guides the book, "What sort of regime considers the absence of people preferable to imperfect people?"

In the book (written with Christophe Bataille, translated by John Cullen, Other Press, 271 pages, $22.95), Comrade Duch, the master torturer and commandant of Phnom Penh's prison S21, from which no one emerged alive, explains to Panh, "The Khmer Rouge were all about elimination. Human rights didn't exist."

Panh was 13 when the Khmer Rouge took over. A happy childhood gave way to nightmare. He watched his whole family murdered or starved to death. With other child laborers, he worked in the rice fields.

Later, it was his job to drag corpses from an infirmary and dump them into a pit he and other kids had dug. After the Khmer Rouge were chased back into the jungle, he got to Thailand and eventually to France, where he is now a documentary filmmaker. He made "S21: The Khmer Rouge Killing Machine" and "Rice People."

His book's method is to alternate childhood flashbacks with his present-day interrogation of Duch, who, in 2010 was about to go to trial, one of the miserably few leaders ever held accountable for war crimes.

Duch talks softly and frequently laughs. He accepts "responsibility" in a vague way but slithers away from direct questions about what he did. He was just a "paper pusher," a bureaucrat. When confronted with his own papers showing orders for various tortures, he shrugs. "Duch is an ideologue: enemies are waste matter, to be treated and then destroyed. It's a practical task that poses hygienic, mechanical and organizational problems."

One of the prison's rules was that no one could be killed without writing a confession first. If the text didn't please Duch or his guards, it had to be written again. Sometimes Duch would construct his own version of the prisoner's crimes. There was no doubt of guilt; after all, the prisoner had been arrested, ergo …

Panh will have none of the sanctimonious twaddle that holds we are all capable of evil. No, there is much good in the world, but it is a dull and everyday occurrence; it does not fascinate. What makes Duch monstrous is that he is a thinking man. A former math professor, he willingly adopted the Khmer Rouge ideology with its airless, insane but precise logic.

Brigitte Frase is a book critic in Minneapolis.