I'm starting to think Chicken Slayer-kun's blood had some anti-chicken thingy in it, but that would be too cliche.
But then again, we're treated with the classic "MUH REVENGE AGAINST SOCIETY" villain, so
I don't think there could have been an actual happy ending. Society had ended in Japan. All the adults are dead. All children will turn into chickens then they grow older, if the mad scientist won't agree to produce an antidote. No country would accept infected children. So what are they going to do about the naked girls who are hanging by their nipples?