I let it live too but omg when i made the trip to collect the reward, my body was not ready for what i saw >.>
I dont even know if the spirit did what he promised.
BTW the witches design almost made me vomit, that one with the eye full of flies.
I know but it seems that same scene plays even if you do kill him.
And yeah, if you kill that thing it will keep its promise but it does something else later on wont say no more lol xD