It has been 31 years since the Khmer Rouge were forced from power in Cambodia. During their four-year reign of terror, as many as 2 million people, or nearly one-third of the population, were killed. For over three decades, there has been no reckoning for the perpetrators of these horrific deeds, no justice for the victims.

That changed last month when Kaing Guek Eav was found guilty of war crimes and crimes against humanity and sentenced to 35 years in prison. The verdict offered some closure to the survivors, but not much. Justice requires rationality, some sense of cause and effect: The Khmer Rouge crimes defy that.

Kaing Guek Eav was a math teacher when he joined the Khmer Rouge in 1967. He took up the nom de guerre "Comrade Duch," and ran prison facilities that the Khmer Rouge set up in territory they controlled. When the party took control of Cambodia in 1975, Duch became head of the Tuol Sleng detention center in Phnom Penh, known as S-21.