She also could have killed her when Koro was crushing Mine instead of slicing of the arm of Koro, that way Koro would have died as well, 2 with one stone so to speak.You know sheele would still be alive if she was a better killer.
She had already sliced off both of seryuu's arms and hard her in panicking mode. Then for some reason she sits there and let's her call Koro.
She had the perfect opportunity and decapitate seryuu but she hesitated after chopping her hands off. Why didn't she finish her off? She just walked away, and that was her downfall.