Usually there is one instigator. Remove him and most likely your problem will be solved. One usually get things started. I kind of had this happen recently except I think one of the males in my bachelor coop and pen was injured either because he was knocked off of the roost or because he fell off of it, either way the other cockerels started beating him up. I saw them standing over him and as soon as he would move they would attack him. I took him out. No more fighting. I also re-homed the rest. Just my opinion.
I am very curious if you ever figured out what it was killing your chickens.
I don't know why. I have had the same things in the past. One time it was 2 of my older hens beating up on the pullets. I eventually took the hens to an auction. Recently I had males around 7 months old beating up on one male. He had somehow hurt one of his legs and was hopping around so I think the others sensed he was hurt and took advantage of him. I took the male out that was being beat up and put him in a cage until he healed. In the mean time I found someone to take the other males. I still have the one that was being beat up, he is well now but he is going to go too as I need the space he is in. One time my Buff Orpington girls were ganging up on a BO gal in their coop and pen. They had her almost featherless. I took her out and put her in with my RIR's and they accepted her. I had sprayed her with Blu-Kote so she was blue. When she grew in her new feathers she was beautiful. Sometimes if you put the one being picked on in another coop and pen it works. I wasn't sure what would happen but it worked for me. I have completely switched the birds around and put them in different coops and pens then the new coops and pens were new to everyone and the pecking stopped. Good luck. I hope you find a solution.
