why didn't hitsugaya kill yukio

Do te see Toshiro as a murderer? No, Toshiro wouldn't kill somebody who was weak and was already captured da him. He let him go, because he Lost all of his power (or at least, Lost 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 secondo one is toushiro has a deal with yukio that if he return everyone in the outside but no one need to die...