User blog comment:CoolJazzman/Was Aizen wife Tosen friend?/@comment-26420452-20131110213749/@comment-28861328-20131111024254

There doesn't have to be a "reason" he turned evil. Aizen literally just did things that amused him. Hollowfication experiments, screwing with people's heads, making plans that spanned centuries upon centuries. That's just who the guy was.