dead hen!!!

[Kinder89]

We bought 8 hens and a rooster when they were about 2months old. Everything was fine until they got older. Then out of nowhere the hens started to mount one another and pluck the feathers from the backs of the little hens. Now, tonight, they killed one of the hens. Her neck was snapped and after burrying her, some if the hens began to attack another hen. How can I prevent them from killing another one? I'm upset and anger that they did this when they never have before. Please help.

 

[Impress]

How large is their coop and run?

 

[Kinder89]

Their coop is 56 sq ft and they have an acre to roam

 

[Kinder89]

And they are only in the coop at night. They coop up on their own and are let out around 8 am. They fight constantly but not as violently when my dogs are outside.