Rats can kill chickens if they get into the chicken coop at night and then bite the chicken.
The rat kills a chicken or kills chickens by biting the head or neck of the chicken which can cause the chickens to die as a result of bleeding to death.
However the good news is that mostly rats will stay away from adult chickens but they do more prey on chicks and the eggs.
Make sure you put out rat traps around the chicken coop to avoid getting rats into your chicken pen or chicken coop.
If you can hopefully catch the rat before it gets into the chicken pen or chicken coop then they will not be able to kill the chicken.