I think it is death by flockmates. In my opinion, if it were a wild animal that had that much time to eat the guts carefully from the vent, it would have spent more time just killing and eating the best parts of each chicken it could grab. Were there other scratches or claw marks on the bird? Was the head/throat mangled? What were the other chickens doing when you found the dead bird? Did they seem interested in the body?
I'm sorry for the loss of your little bird, but it does look like your other girls ate her.
I'm sorry for the loss of your little bird, but it does look like your other girls ate her.