When integrating flocks

How much aggression is too much?  Do I have to cull the aggressor?

Well, I think it depends. Is the aggressor another hen or a rooster and is the one being picked on male or female? How many birds are in the flock? If the flocks were just integrated, the females must reestablish the pecking order, and that may take a while and poor behavior from a human perspective may subside. If it's multiple males, one will try to be dominant over the entire flock, and likely will bully other males relentlessly.

It's one of the older hens.  I have a rooster in the younger flock, but I'm culling him (eventually...).  There are 5 older hens and 7 younger ones; 4 of the older ones pretty much ignored the younger hens when I tried to put them in the run (625 sq. ft.; plenty of space).  But this one hen actively goes after the younger hens whenever she catches sight of them, chases them away from food, etc.  She might have pulled feathers out, but no pinning.  I let them head back to their separate coops after that.  I'm seeing the wisdom in just fully replacing the flock every couple of years...  I'm not really set up for that, though, and my kids have named half of them, lol.