I've always been of the mind that over-aggression is genetic at the root. If you have a bird that has it, he's got it for good. That's why I've never tolerated it. I don't want any of that business passed on to any offspring. And, heck, I ain't spending what I spend in feed to go out and get whooped by a bird with a bad attitude.
I raise mine hands-off. I've heard it particularly advised against to hand-tame cockerels from a young age because of the potential boundary issues it may cause. I don't think this necessarily causes aggression problems in and of itself but could make the aggression worse if the bird is hard-wired that way.
I've always raised mine the same way and had variations in temperament, even within the same breeds. In the line of Wyandottes I'm working with now, docility is one of the traits that has been taken into account, and the roo I've got is one of the favorites I've ever had. He likes chest rubs at roost time, which is about all I give, and doesn't pay any attention to me otherwise, except at feeding time of course--which is how I like it.
