If this is actually Obito not just body/eye then his reason for having this drastic change might be that he just grew tired of seeing all the death ( family?/friends) and unkept promises of peace. It will probably have something to do with Rin and here apparent death. If his actual plan is infinite tsukuyomi then it's plausible to reason in his eyes this is the only way to create "true peace" by having everyone in check. I believe he wouldn't see himself as the enemy but the hero that brought peace, admired by all. In a flawed way of thinking created by tragedy. Similar to Nagato's way of thinking. Nagato wasn't pure evil. He was changed by the tragedies in his life which caused him to see life and death in one way no matter who's opinion, he was going to accomplish his version of "true peace." Well until he had a chat with Naruto.
This would be my reasoning behind him actually being Obito.
This would be my reasoning behind him actually being Obito.