What Maul was, I believe, was someone brought up purely to be a killing machine. Palpatine was trying to create the ultimate apprentice-someone who would do his bidding, bring down the Jedi, etc. Unfortunately, because he had had Maul since he was young, Maul was very one-dimensional. He only knew murder and darkness. He didn't care about anything else. In a way, Maul is kind of tragic, because he was basically Palpatine's slave creation thingy. So then Palpatine thinks, dude, maybe I should corrupt someone who's already been around, rather than work with raw material.