Next manga will probably have a few scenes of what will be the insight as to what happened between Obito getting crushed up to when he joined the Anbu.

I'm willing to bet, he had a rebellious time in his life where he was not exactly what you would call "orthodox" in that he went, I don't want to say evil, because thats too black and white, but he wasn't exactly white if that gets my point through.

He didn't just sit in Konoha for what is it, 5 years, after Obito got crushed, I mean think about it, he tried to take Sasuke under his wing and teach him revenge isn't worth it. Normally only people who have gone through failing with their revenge or succeeding and then realizing it wasn't worth it can say things like that.