My hen only flock still chases the leghorns I added last spring and refuse to mate the roosters. Sometimes its just easier to maintain separate flocks or get rid of some I agree!
Inadequate protein,too many treats,not enough food, too little space or lack of privacy or hiding places are a few of the most common reasons they become aggressive. A coop that provides each bird 4 sq foot and 10 in the run is usually sufficient.A photo of your set up would help.