He's a human who's altered himself with an overwhelming amount of forbidden techniques (he desires to know every last one of them) which made him next to invincible; having only one weakness, genjutsu, or the wearing away of his possessed body when it starts to reject him. I think he had a solution, or at least implied he did, for when he would take Sasuke's body, he would have it forever. The reason he's wanted Sasuke so bad is to conquer his one weakness. He really never counted on losing to the Sword of Totsuka, which, ironically, is in itself, also a Genjutsu. Orochimaru sought to attain immortality (living forever) but first wanted to make sure he was immune to being killed (part of being immortal). His fight for Genjutsu led to his 'apparent' downfall. Knowing how intelligent he was, I can't confirm his complete annihilation from the Narutoverse. I do admire his ambition though because he really came close: being immune to physical attacks from even the Kyuubi and Tsunade. That is pure amazing.