Do Ты see Toshiro as a murderer? No, Toshiro wouldn't kill somebody who was weak and was already captured by him. He let him go, because he Остаться в живых all of his power (or at least, Остаться в живых Ichigo's power), so he just let him go, knowing he wouldn't do anything else bad.
because toushiro is not that kind of person that will kill other people because they are his enemy and the секунда one is toushiro has a deal with yukio that if he return everyone in the outside but no one need to die...
