It's almost impossible to bring them to account and even then it happens again, and again, and again.
At least some of them do face justice. The International Criminal Court, the International Criminal Tribunal for the former Yugoslavia, and the International Criminal Tribunal for Rwanda have all indicted, convicted, and sentenced war criminals. Khieu Samphan is serving a life sentence in Cambodia
It's all about who they are and whose backing them and what the vested interest is though. Gah, I'm such a cynic.