Quote: Have hope and keep trying.
In my experience, this is normal behavior for artificially bred, reared and housed chickens, or the offspring from those that descend from such a recent ancestral environment and social structure.
They've lost the instinct for the natural family group after being reared without mothers, fathers, and a normal varying-age group flock structure for many generations. The instincts will come back if you let them, though it may well take generations. But you'd need to breed them naturally, and separate or not breed those which show absolute intolerance and violence to others. Some of your current flock may never adjust to other birds, as they often can't outgrow the faulty instincts in one generation.
Getting birds from a hatchery often means you get birds which are only tolerant of their own age group, gender, and type. Birds from any breeder who separates flocks and artificially rears them are rather often behaviorally/instinctively confused and aggressive to others.
This isn't a problem I ever have with my non-hatchery stock. Injured or ill birds are not harassed, babies range unmolested with mothers among the whole flock, roosters get along without fights, it's all very peaceful. It IS possible. But I remove or rehome bullies and violent birds, and won't breed any that show negative mentalities. So if you're not keen on doing that, then you may just have to work around your resident bullies/killers indefinitely.
I'm not saying my way is the only way, but it's best for me; what's best for you may be different. I need my flock to freerange in peace for health and productivity's sake, I won't keep birds that harm others or won't get along. Genetics have a lot to do with this, but so does environment and rearing.
Both genders can get along with others of all breeds and ages. All you need to do is remove any that absolutely will not tolerate others, or at least don't breed them, and breed those that do. In a few generations your flock will regain instinct.
Best wishes.
