When you find a chicken in your chicken coop with it's head bitten off the culprit could be a few different animals that bit the heads off chickens.
The three animals that most commonly bite heads off chickens are raccoons, weasels and owls.
Also some other animals that can attack and kill chickens and even might bite the head off of chickens are foxes, coyotes, bobcats, feral cats, hawks and dogs.
You need to watch out for all those animals especially when you're raising chickens and I would recommend building a good strong pen for your chickens to reside in.
Also having a top on the chicken coop or chicken pen can help to prevent animals such as chicken hawks or any other flying animal from swooping down into the chicken coop and taking off with your chickens.