They’re intelligent and very instinctive birds from birth- that doesn’t mean that they’re vicious though. My chickens that I’m raising are all sweethearts. It’s all a matter of their environment. If you overcrowd them in an indoor factory farm of course they’re going to turn on each other, they’re extremely stressed out. Chickens that are raised outdoors with lots of space and different kinds of food are a lot less likely to act up and turn on each other. You can even taste a noticeable difference in eggs from happy chickens.
Roosters on the other hand are usually fucking assholes.