I have a theory..
Gin has no soul. He kills shinigamis and takes their zanpaktuus --> explains why ichigo could not read him when fighting.
possible motive :
- spirit king is the one responsible for gin not having soul and he wants to take it back by force killing him with Aizens sword
plot : he reveals all of it to ichigo who sympathises but bests him in battle and then also defeats and kills spirit king and replaces him.
twist:
the spirit king is urahara kisuke.
umm..no flames please
