I dont know why but i always tought orochimaru was a clever guy, you know he even developed the only known existing jutsu to reach inmortality. But then he trains sasuke to get stronger than itachi, because he fears itachi (so he wants sasuke to be stronger than the man he fears), and hopes that sasuke kills him and then.... submit himself to orochimaru to take over his body, and he hopes this to happen? I Suppose that i overstimated orochimaru thats all.