Why...

[narutoblitz]

I submit the humble theory that Obito was not Tobi and didn't "turn evil". I think Zetsu took over his consciousness and mind rape'd him after Rin died. It would explain further why Obito acted so silly at first as the false persona Tobi. I think he's currently struggling to regain control of his body from the Hashirama DNA and Zetsu.