The purpose of keeping a rooster is to help protect your flock of hens from predators as well as to mate with your hens and fertilize their eggs. The last thing you want your rooster doing is attacking your hens. Roosters can attack hens for a variety of reasons, and you should not necessarily assume the hen being attacked is sick unless you see other signs of illness.
Roosters and Hens
A rooster will naturally dance around a hen or shake his wings before mating. This is normal behavior, so don't be surprised by the ritual. Some roosters are a bit rougher than others during mating, and a little bit of noise, chasing, light pecking or general roughness is not out of the ordinary, especially if the hen is not altogether interested in mating at the time. However, with that said, a hen should not be bleeding or otherwise injured after interacting with your rooster. A hen who is visibly injured after mating or otherwise interacting with the rooster needs special attention if you want her to survive long-term.
Roosters have been known to attack hens that are ill. Sick hens can be a danger to the flock, may smell strangely or otherwise set off the rooster's natural instincts to protect the rest of his girls. Remember that attacking a sick hen is not guaranteed behavior that all roosters exhibit. A rooster may attack one sick hen but not another. He may attack a healthy hen that he simply does not like for whatever reason. Some roosters may never attack any hen while extremely aggressive roosters may routinely attack them. Knowing your rooster's behavior will help you determine whether his actions are normal or out of character for him.
Signs of Illness
An out-of-character attack on a hen combined with other signs of illness from that hen can be a definite sign that your rooster is intentionally attacking a sick hen. A hen that is lethargic, has stopped laying eggs, is laying flawed eggs, is suffering from diarrhea or has stopped eating or drinking is likely ill and needs attention.
Handling the Situation
If your hen displays signs of illness and gets attacked by the rooster, you should separate your rooster from the hen until the hen has recovered from her illness. If the hen is no longer being attacked once she has been confirmed healthy, then it's likely the illness and the weakness it created in the hen prompted the attacks. If your rooster has attacked a hen who shows no signs of illness, you may want to separate her from the flock so you can have your veterinarian check her health before gradually reintroducing her to the flock. If your rooster continually attacks different hens, then he may simply be aggressive and need to be separated from the flock altogether.