Two models of individual evolution
The difference between Aizen and Ichigo may lie in their view of the distinction between hollows and shinigami.
For Aizen, they remain opposites. They can be made to coincide in one being, but they will remain opposities – tearing this being apart, making it weak (cf Aizen’s negative view of vaizards and arrancar). To advance, one needs to go BEYOND the distinction. Something altogether new must emerge in evolution – neither a hollow, nor shinigami. Something beyond these boundaries, beyond “reason”.
Gin could not kill Aizen as his actions remained within reason; while Aizen went beyond it… But apparently did not reach whatever would lie beyond. In the final battle, Aizen does not behave like one who transcended reason.
Ichigo, on the other hand, discovers that the shinigami power and the inner hollow are ultimately one. And he can unite with the hollow/zanpaktou. Why was he able to do it and Aizen was not?
- Aizen thought he reached the limit, as he conceived shinigami to be separated and limited from hollows. So he turned his attention elsewhere
- Aizen never relies on anybody – including his zanpaktou. His relationship with the zanpaktou may be less dramatic and intimate than Ichigo’s. Note that Aizen says “MAYBE that’s the final form of the zanpaktou…”
- Paradoxically, Aizen’s strenght may have played against him – the zanpaktou never needed to take inititative, never needed to save him.
Insofar as Aizen wants to go beyond the powers which remain in him, he is divided; while Ichigo manages to maintain unity. This goes along with the second great theme: rejection versus acceptance of oneself and reality.