What happend? Some times roo's will fight each other for picking order and mates. We they get started They can tear each other up really bad, but I never had one of them kill another one. It is very possible it could happen, especially if it is a younger Roo that is going up against a mature one. Is your roo still breathing? If he is bring him inside were it is warm, and try forcing some gator aid or pedia lite down him along with a small part of a asprin. This may help getting him to perk back up. Your hens will be fine, roo's dont get agressive with hens. If you have any other Roo's I would try and seprate them, give them more room, and try to add more hens so there isnt as much need for them to be so copetitive. If they have spurs you need to remove them. That helps both with them hurting each other in fights, and keeps them from hurting the hens while trying to mate. Once I did have a young Roo that died because the older Roo's wouldnt allow him to get near the water or feed. Our nieghbor gave him to us, but didnt know we were going to be out of town for the weekend. I was going to put him in with my pullets in a seperate pen. By the time we got back he was dehydrated, and was to far gone to save at that point.