Though Aizawa did his best to avoid humans, every once in awhile he became too enticed and knew he had to satisfy his thirst. And you just so happen to be the one currently causing him to lose focus on his duties.

So he decides to get it over with, sneaking into your room while you're asleep. He knows it's not fair, but if he doesn't do this his hunger will quickly consume him and you will end up hurt because of his own negligence.