I don't think the typical chicken keeper interested in eggs and pets, and not free ranging, needs a rooster at all. Life with just hens is quieter, peaceful, less feed, and such.
If they range around, roosters are useful for their alerting behaviors. Hens don't really keep an eye on the sky. They're safer when they're conditioned to respond to alert calls (some is natural, some is learned from roos who always tell the truth - yes, some do lie! About treats too, it's been studied)
Due to my many breeding projects, we've sorted about a hundred cockerels so far this year alone. I'm getting better at seeing aggression sooner, and a big part of that is having a firmer mindset and not making excuses for bad behaviors. It might just be a moment here and there but it says everything about them. As a result the roos we have kept all have a good demeanor.
In my opinion, mean is mean, so if I see a cockerel being excessively nasty to the pullets (even before mating age ) he's on my poopoo list. If I see it twice, he goes to the jerk pen that gets given to a local for food (I can't process atm).
Cockerels in general can be rough on the pullets. The difference to me is if he grabs her neck feathers and hangs on as she scrambles away, often coming up with a beak full of feathers or with a pullet screaming as he tries to stomp all over her back. A good cockerel who's just feeling his oats will let go if she is desperate to get away. It may not sound very different here in writing, but it looks a lot different in real life.
Another thing is pecking her on the head. Some cockerels are trying to mate for the dominance alone, and may suffice themselves with cramming her into a corner and pecking her head. Those boys don't get a second chance, that's how roos kill hens.
Typically, I believe the girls when they render an opinion about a boy. If they have decent, healthy & confident choices, they won't pick a jerk. In the wild they have those choices, but domestic breeding often doesn't let hens determine healthy behaviors.
As for human related behavior, I feel like it's really obvious well before an attack happens. The mean ones have a hard look in their eyes and you will get a lot of glares before they try anything. They all go, there's just no reason to perpetuate that kind of stupidity (I really think it's a lack of intelligence that leads to poor target selection).
One thing I don't mind much is boys fighting each other. It's a very natural behavior that has a selection purpose, and is related to how hard they will fight a predator. By the time they're old enough for breeding, I've separated most of them so it's not a big deal here.
In summary, I really think that comparison is the best way to classify cockerel behavior, and since most people don't have the numbers to do that, asking someone with experience to come for a visit would be an ideal solution.
