I don't necessarily feel he's a psychopath. Though I agree he isn't exactly a hero but he has noble ambitions and whose to say he would be a bad ruler? Several rulers in our own history murdered their way to the top and followed it up with the complete execution of their political enemies and they are regarded as great men and woman who changed their nations for the better. Whose to say Vayne wouldn't be the same, I don't remember him talking about turning Dalmasca or Nalbina into the FF version of the Congo.