I remember a Web journal or disc extra where Lucas is talking about Darth Sidious/Palpatine and says something along the lines of "he's the Pure Evil. The Sith Lord who's out for power simply for power's sake. It doesn't matter to the story what motivates him, that's not so important. All you know is that his life has brought him to this point where he wants galactic domination."
Sorry, but: that's just lazy storytelling.