Board Thread:Questions and Answers/@comment-32617252-20180806022624/@comment-5171765-20180807170632

It was never said in any databook that Aizen killed Tosen. The only way you could say that is to say Aizen is responsible for making Tosen whatever he was (with its inherent risk of soul suicide). Tosen died because of Soul Suicide, not because Aizen killed him.