Obito is a bitter and disillusioned idealist who seeks what he thinks is peace using whatever possible means. In his twisted ideology, he believes he is making the world a better place.
Orochimaru, despite the obligatory painful back-story regarding the death of his parents, seeks something only for himself using whatever possible means. In his twisted ideology, he only cares about achieving all the knowledge/power he can for the ultimate goal of his own immortality. He's the one you gotta love to hate.