Process of elimination. Remove the nasty hen - put her elsewhere or truly eliminate. If there are no additional problems - then problem solved.
If not, then there's the predator issue. It does seem strange that only one chicken was attacked and killed... and left there. I'm told that weasels bite off the heads of the chickens, so probably not a weasel. Rats might be a possibility.
If not, then there's the predator issue. It does seem strange that only one chicken was attacked and killed... and left there. I'm told that weasels bite off the heads of the chickens, so probably not a weasel. Rats might be a possibility.