Complex sentence: Kaname Tōsen interrupts and brings Grimmjow back to be punished; since Aizen refuses, Kaname severs and incinerates Grimmjow's arm so he'll be demoted.
Split into simpler sentences: Kaname Tōsen interrupts and brings Grimmjow back to be punished. When Aizen refuses to punish Grimmjow, Kaname severs and incinerates Grimmjow's arm so he would be demoted.