User blog comment:CoolJazzman/Was Aizen wife Tosen friend?/@comment-26420452-20131110213749/@comment-26420452-20131111071637

Aizen turned evil when he realized he was exceptional. That's what the talk between Urahara and Ichigo implies. God, I'd love to know his backstory.

And even though his plans really took time to be realized, it's not that long if we put it in the context of Shinigami's lifespan.