What happen to the whole promise Ao made to Nezumi to kill everyone at the Org so that she wouldnt have to keep following their orders and could live a happy life together? I know Ao was involved from start which is why he made contact with Nezumi but I believe he clearly loves her...
Also the amount of opportunities they both have had to kill Iruka and haven't is a headache, I mean Nezumi wiped out like a 100 dudes in a laundry room for Christ sake.