Susanna, that is totally unacceptable behavior. You need to immediately separate that roo from the hens. You need to keep him separated until you either kill him and eat him, have some one else kill him, or kill him and bury him, preferably planting a lovely rose bush over the spot. Occasionally, a roo or other flock members will drive a sick bird out of the flock. But for him to kill her, that goes into the realm of truly deranged. Tell me a bit about your set up. How many birds, total. What are the dimensions of your coop and your run? It may be possible that if your birds are shut in a coop/run situation, the hen can't escape the roo. No matter what the coop and run size. That roo needs to be killed. He is a killer, and should not be allowed near ANY flock.