Speaking at a UN-backed tribunal in Cambodia, Kaing Guek Eav - also known as Duch - expressed "regretfulness and heartfelt sorrow" for his actions. Duch is accused of torture, crimes against humanity and premeditated murder for his alleged role in the deaths of more than 10,000 people.
This is a bit like calling Rudolf Hoss a “key Nazi leader". Duch* was a functionary. He never governed a KR administrative unit, let alone province, and was nowhere near the Politbureau. He was certainly not as senior in the KR hierarchy as Hun Sen, the country’s current Prime Minister. The other three on trial with him were very senior KR leaders, but Duch is the only one charged with anything specific and the only one to admit anything. Preliminary trial proceedings were dragged out for an age, apparently in the hope that the former senior KR detainees would die of natural causes before possibly saying anything in court. It would be difficult to just dispose of them with the UN poking its nose around the place.
S 21, the extermination camp Duch ran, was the place where cadre who fell foul of the Khmers Rouges constant rounds of internal purges were sent for torture, confession and disposal. If Hun Sen and other former KR eminences in Cambodia’s ruling party hadn’t bolted for Vietnam at the right time, Duch would have got his hands on them, too. Now they have him. There's obviously general justice in his prosecution, but it doesn't seem to go much beyond the requirements of the current Cambodian leadership for revenge.
*pronounced "doik", apparently. And for those curious about such things, here's the Khmer Rouge-imposed Cambodian national anthem.