Orochimaru is just a snake lover...period. Nothing much else to say. I mean, he defeated the Third Hokage, who was as old as dirt (like 70 or something) and hardly had the chakra he used to have. Orochimaru couldn't beat Naruto (the only reason he survived was because of hiding from the fox), couldn't beat Itachi (because Itachi is that awesome), and couldn't beat Sasuke, because he's too stupid to know that Sasuke is actually a lot like Itachi and his Sharingan has become very powerful. At least Madara and Pein have an actually goal. Madara wants revenge on Konoha and the people who have wronged him, and Pein wants peace (he claims at least). Orochimaru is just a creepy, inhuman character in the series that just gets annoying after a while. Orochimaru isn't strong enough anyway to be the primary villian of the series (that title goes to Madara and Pein).