Orochimaru ambitions were to
1: Learn all the jutsu's in the end
2: Immortality
3: Crush the 5 major villages (killed kazekage and hokage)
4: Obtain the sharingan (and presumably the rinnegan)
Tobi
1: Create a world of an infinite genjutsu
Orochimaru's reasons
1: His parents had an affect on him when they died
2: He had a cruel outlook on life
3: That snake shedding he saw with hiruzen influenced his ambitions
Tobi's reasons.
1: Rin died
Orochimaru was always the creepy one who even if he isn't as powerful as lets say madara, will always be remembered as the villain that everyone feared. No one dared to cross his path, even the akatsuki couldn't kill him. Matter of fact even sasuke attributed his death to the fact that he was in a sick state. Its near impossible to kill him since he can just come out of cursed seals. The man is the definition of frightening. While Obito on the other hand......
Someone I loved died, now its time to ruin the world.
I'm not a tobi=obito hater, I just wish it was more dramatic and more reason was given as to why he wants to take over the world. Orochimaru's story was rather touching as his parents died and he feared death.
