We had something kill two young chicks (about 6 weeks old) a while back. These chicks were in a secure coop. One chick we never found. The other was dead on the coop floor with its insides eaten out like you described. We have hardware cloth and buried dog fencing on this coop and saw no evidence of a racoon or other predator tearing into the coop. We figured it had to be a rat or a snake. The cats have taken care of a few rats and we are putting out the plaster of paris and peanut butter balls in places that the chickens and pets cannot get to them.