User blog comment:Xilinoc/Ch. 605 - Don't Call My Name/@comment-5693502-20141113174043/@comment-26420452-20141113201018

Aizen probably wanted to execute the Soul King and make the whole universe a dictatorship of the proletariat, and then a socialist utopia.

On the serious side, if Aizen were to cooperate in the Soul King's protection and things went awry with him eventually, nobody could handle him anymore up at the Palace. The Royal Guard and Ichigo would be exhausted by dealing with Yhwach and his underlings.

And yes, Kenpachi was the only one who could defeat Gremmy, but how could Shunsui predict that there would be a kid so OP he could make a meteor appear above the Seireitei?