Obito is actually one of the most complex and misunderstood characters in the series.
Try this analogy. Why is Spider-man saving people around ? Because uncle Ben was killed by a criminal ? No. Because he doesn't want innocent to be caught into what happened to his uncle. Differentiate the trigger and the reason. Rin/Uncle Ben death is the trigger. The reason for Spidey and Obito is that they are both pissed off .
Whenever Spidey is down on his resolve, he goes back thinking about his uncle's motto. So does Obito when he needs power surges he remembers how things were back then.
He himself said that he dislikes the shinobi system and mindless killing.
Obito is moved by purpose, even as child, he showed that he will act only according to what he think is right...so fighting naruto, caused a doubt in his heart/ideals....so Obito could not give it a 100% cuz he wanted to be assured that what he was doing was indeed right. By trying to break Naruto with questions like why do u fight? He was actually trying to convince himself that he is indeed on the right path so he can go all out.
As i said, Obito is not your usual villain(orochimaru/madara) who will do shi*t just for the sake of more shi*t. Just cuz he chose a wrong path doesn't mean he had to stay wrong till the end, he is willing to accept his mistakes even at the last breath and change his ways. This is why it is far more important for him to stay true to himself unlike other villains. He does not take action unless he believes in it.
