It's a combination of genetics and husbandry. I stated before and will again. I can make just about any rooster a man-fighter. Genetics can impact how much effort I must invest.I think it's also counterproductive to the entire concept of domestication of animals. The whole point of domesticating something is to breed "domestic" traits into it, like not attacking humans. Breeding an aggressive rooster is, I think, a step backwards.
There was a study done, in Russia I think, on domesticating foxes over a long period of time. They basically only bred the friendly ones, and now they're all friendly, I think that showed genetics can play a large part in behavior.