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.
I own chickens and have had a bullying problem in the past. In the winter they have a lot less space and they get bored and stressed easily. The hen that took the “rooster”/protector role started pecking the smallest and sweetest chicken and drew blood. I had to keep the small chicken isolated from the flock for a few weeks while its wounds healed and put special goggles on the bully chicken for a month- they prevent it from seeing in front of itself so it forgets what’s there after a few seconds. Yeah they can be vicious, but it’s definitely preventable if they’re raised right instead of at a factory farm.