Hello this just happened to me as well. I have the meanest rooster he is just nasty and mean. I want to get rid of him but my son says there must be a good reason the rooster wants to kill the hens. Maybe they are sick (they don't appear sick) otherwise I have no idea for it. If you find out the answer please let me know. Thank you and good luck.
More thoughts on why roosters kill some hens:
1) they are possessive and jealous. A number of my most abusive roos/cockerels started out as my special sweethearts; each one stood out from its hatch mates as a darling who loved, and couldn't be separated from, its caregiver. As they matured, they tolerated, less and less, other chickens encroaching on their territory (me/FOOD); their targets were smaller and weaker, older (and weaker), assertive eaters, favorites (mine), female. The nastiest punishings seemed to occur at feeding times, around their FOOD and (THEIR) provider. I could say the Roo was being territorial and asserting his dominance, but I am talking vicious here, more than territorial and dominant. I am talking ENRAGED. I DO NOT think this is normal, I think this happens because I spoil and mismanage my chicks, especially my most darling boys.
2) they are racists. OK, OK, but you must remember that chickens are VERY color sensitive. IN GENERAL, my SS Roos go after (as if to kill) my LS hens, and my LS cockerels do the same to the SS hens (and other dark hens.). Yesterday a Black Sex Link pullet came flying through the open patio door, with an LS cockerel in heated pursuit. I put her back out, and she was immediately attacked, not by just the one LS.cockerel, but by 3 or 4. So I let her back in, and she refused to go back out on her own all day; the LS cockerel ranted, paced, and flew at the door all day long trying to get at her. This was my sweet babydoll Roo who ADORES me! It did not look like amore to me. (And there is that rare exception).
Ok, so does this all sound a bit anthropomorphic? Maybe it is, but chickens are funny, funny, funny. What I am really trying to say here, though, is that chicken behavior may to some degree be a reflection of - or response to - our own ways of interacting with them. That is not to say they are "bad" and that I am "wrong" in my indulgence of them. The relationship between humans and chickens is ancient, it's not going to change, and neither am I. It just IS. What I can change is my management.
So for the present, I am separating the sexes. I made a large rooster coop for the roosters (they really do well together in a coop of their own) and, with the exception of breeders, all the hens are going into the layer coop. Supposing there is safety in numbers, the white fowl just MAY get along with the dark ones. (They segregate meticulously). And I am considering using Broodies instead of incubators to improve my stock. (I got a stellar BA mix Broodie from last year's hatch, and I've noticed that the chicks she broods are different in character from the ones I raise).
Roosters are strange IMO. It is really hard to figure them out. Here is one to figure: I have an LS roo who is obsessed with my big (huge) white and black Saint mutt female dog. She is a chicken chaser (?herder), but every evening she goes down to the gate to his pen, and he follows her everywhere. Then she lies down, he dances for her, nicely, and he hops on her back, grabs her neck fur, and treads on her back. She likes this Rooster! She will roll over and gently push him off, then come back and lie down again. I don't get it. Every evening they do this little game, a 7 lb. white and black rooster scratching and treading the back of a 130 lb. white and black dog. No animosity, no territorial ism, no bad vibes, just mutual acceptance. But don't let a Speckled Sussex get near - either of them.
Funny, funny, funny.
I do have a loner Roo or two that hates hens. They are Speckled Sussex, and seem to hate any hens getting near them or their territory, they don't go seeking them out for the most part. I can't imagine why they behave this way; maybe they'd be different with a flock of their own? I have read on a BYC thread that chickens get their dispositions from their mothers, so tossing a quirky Roo may not be effective as not hatching anymore of its dam's eggs. More interesting than TV!