The premise is kinda dumb considering that "demons and humans have been at war for millennia" or whatever. He's really going to heavily strengthen the race that will massacre his own people because, why? Yes-- because he's a teacher of course.
I've only read up to this chapter so if they touch on this a little more, then awesome-- but the fact that he acted so happy-go-lucky after being fired but is still willing to sell out humanity right after, is really weird. I'd understand why he decided to teach them if he were pissed at everyone, but he's clearly not so the disparity is very jarring.