I don't see how Delita is really a psychopath. He spared Cid and his son despite both of them being a liability for his own plans. In the end, Delita mostly took out people who really deserved to be taken out and as VeloZer0 said, he's remembered as a hero and wonderful king, so he probably ran Ivalice better than anyone else. Besides, Ramza had bigger problems than Delita.
Ramza is trapped in a scenario based in reality. Killing Delita would have only led to more suffering and the possibility of a greater tyrant coming in control. I love the fact the FFT portrays its story in a more realistic context with the total absence of the usual RPG "black and white" morality scheme. It creates a richer more philosophical take on the story telling.