There's no doubt about it. If he's compromised his morality enough to get something out of killing animals, even small ones, then all bets are off. He'll amp it up, slowly, and ease his way into more heinous shit every time. I don't think he's going to take the plunge and Patrick Bateman some hobo in an alleyway tomorrow, but he'll work his way up, all the while convincing himself "this isn't that much worse than the last one, it's fine/just one more, it's fine", things along those lines. I think within the decade there's a high likelihood he'll take a human life.