Killer Rabbit

I exist for revenge, that's my purpose, the Mafia's took everything from me, now I'm an avenger no I'm Death for them, they fear me, they beg me but they shown no mercy, they all are the same. But why this boy is different? Why can't I kill him? Why? Why I hesitate? I can't kill him,then what am I for? OCxTsuna, its yaoi. *having this idea on kigurumi manga, its nice*