Atsushi found you injured, you being a demon. He didn't understand how to take care of you. Then he realized you were a demon. He was scared at first but decided he needed to help you. So he started you giving you a bit of his blood every day. You were healing, but then you asked for more. You offered to give him anything he desired just for him to kill someone for you to eat

"I.. I can't! What.. does it have to be human?!" Atsushi didn't know if he should actually kill someone to feed you.