Board Thread:Naruto Discussions/@comment-2198188-20140421004522/@comment-4729034-20140424074820

One fatal error in your summary is that it had absolutely nothing to do with Kakashi personally killing Rin that made him into Tobi. When Madara questioned Obito's intentions of the Eye of the Moon Plan, he said he would have both Rin and Kakashi in the dream world. That's also why when Kakashi questioned as to whether or not Obito blamed him, he said "what good will blaming you do?" and then proceeded to talk about the worthless reality and so forth. Furthermore, whilst in the pocket dimension with Kakashi, he revealed the truth of her death and Kirigakure's plan to use her. He knew everything about it and why, but he didn't care. It was simply because she was no longer alive that he went down the path of darkness. Well, that coupled with the cruel ninja world and what it meant to be a shinobi.

In a way, Obito held on to his idea of trying to produce a new shinobi world as he promised as a kid, albeit this method was a bit more aggressive and forceful.