Hmm

I thought he just liked killing lol he was bothered the monsters were fleeing because he cant kill anymore... just a thought xD

I think it's pretty obvious that he sucks life force. If he doesn't have enough he falls asleep. So he likes killing and needs to but because the monsters run it makes it difficult and he has to get the source of medicine from somewhere else. But that's just if the story is as cliche as I think it will be. This will prob result in a moral conflict between the mc and ml.