Duch may go first

The defense lawyer for Kaing Khek Iev, known as Duch, expects his client to be the first ex-member of the Khmer Rouge to come before the Extraordinary Chambers of the Courts of Cambodia (ECCC). (Source)

Khang Khek Ieu is infamous. He was known as Comrade Duch and was in charge of S21 prison from 1975-79. During that time, the best guess is that 20 000 people were exterminated under his orders, including the few foreigners who did not evacuate the country or who were captured in territorial waters.

Prisoners were tortured with electric shocks, searing hot metal instruments and hanging, as well as through the use of various other devices. Although many prisoners died from this kind of abuse, killing them outright was discouraged, since the Khmer Rouge needed their confessions... Physical torture was combined with sleep deprivation and deliberate neglect of the prisoners. The torture implements are on display in the museum. The vast majority of prisoners were innocent of the charges against them and their confessions produced by torture

After the interrogation, the prisoner and his/her family were taken to the Choeung Ek extermination center, fifteen kilometers from Phnom Penh. There, they were killed by being battered with iron bars, pickaxes, machetes and many other makeshift weapons. Victims of the Khmer Rouge were seldom shot as bullets were viewed as too precious for this purpose. (Source)

After the fall of the Khmer Rouge, he escaped to Thailand. He travelled to China teaching Khmer and basically got on with his life. He converted to Christianity and even worked for aid agencies. He escaped discovery until an Irish photojournalist, Nic Dunlop, tracked him down:

I was talking to an amputee Khmer Rouge soldier when a short, wiry man appeared, wearing a white T-shirt emblazoned with the initials ARC (American Refugee Committee). Shaking my hand, he politely introduced himself in perfect English as Hang Pin. He was a born-again Christian who had been working for American aid organisations since 1997. He took a keen interest in my Leica, asking me how much it cost. Using this opportunity to photograph the people in the group, I caught him in the frame. Large ears, bad teeth, cropped hair-he had aged a little, but the likeness to the photograph tucked in my back pocket was unmistakeable. Hang Pin was Comrade Duch. (Source)

He is now awaiting trial. Everybody in Cambodia is waiting for the Khmer Rouge Tribunal. There are near constant delays and the accused are old and may not survive long enough to be tried.