Quote from Tjuren
Wait. So Gin became Aizens lackey and betrayed everything and everyone around him just so he could take revenge on Aizen for doing that to Matsumoto? Did he really have to go that far?
I'm sure that was just the trigger. Aizen, as you have seen, is a severely conflicted being, and Gin probably saw the extent of what he may become. I'm guessing at first, he got close to Aizen for those childish reasons, just to see even more of Aizen and decided that he really needs to be eliminated. He says "with this, it would all be over", and notice how he attempted on securing the hogokyu instead of securing Aizen's death first.
Is "all" just that Rangiku assault? Because I think there is much more. Maybe he didn't do it for everyone, but Gin clearly felt the need to see this person eliminated, maybe because he thought along the lines of "if someone did it once, they'll do it again", and that he or Rangiku might someday be in danger once again. which was very true with Aizen, especially with his superior arrogance. This is just speculation on my part though.
I have just gotten back into Bleach after Gin showed up and made it interesting. If Gin is killed off by super saiyan god butterfly man, I probably would never pick up the manga again lol.
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