I'm assuming it's definitely evil because 1. I'm a man. Men don't have babies. and 2. I've never had sex. Now that second one shouldn't matter in the face of 1, but it helps to rule out something like "I'm secretly some kind of alien sleeper agent gathering genetic material for hybrid making" or "I'm actually a humanoid worm and can fertilize eggs i take during sex...somehow" which are pretty stupid explanations anyway, but then the whole situation is stupid, so I'm going with "virgin-born butt baby is satan" and proceeding as usual.
What is usual. Selling that little bastard to science, duh. I hate kids, a lot. Like, in the "I'd rather shoot a syringe full of flesh eating bacteria, Ebola, bubonic plague, and AIDS right into my eyeball than ever be a dad" level of hate, so there is no way this kid was being kept, demonic origins or not. So let the nerds pay me a crapton of cash, then irradiate, dissect, shot, burn, and whatever else the wanna do to the little bastard.
Assuming that I can't prove that I just gave man birth to a satan spawn, dump it off on some chick I know who really wants a kid. Of course I'd mention the whole "Oh, by the way, this thing may be the anti-christ. Like, seriously check with a priest or twelve", but I'm pretty sure she'd take it and it wouldn't be my problem anymore, and unlike just killing it, I don't have to worry about anyone finding the body and arresting me.