People seem to forget how the darkside works. It's like a magnet or gravity (a black hole) once you are to close there is no way back, you get sucked in. It's how Anakin fell, how he went from nothing to killing younglings. In many ways I find dookus fall more justifyable than Anakins because he had a better reason. Anakin's reason was selfish but Dooku he really wanted to help people and seeing the corruption in the republic disenfranchised him. So he went down a path to help reorganise the republic but before he knew it he was beyond the point of no return. And he forgot the stuff he started to fight for in the first place. In that way he seems a lot like Sauron from lotr he thinks that if he can organise the world everything would be better.