Roosters should not normally be human aggressive... but I have read the horror stories here too. I decided to try and figure out why so many have issues... so I decided to research a bit... my findings.
1 do not teach them to roost on your shoulder... this can trigger bad behavior in parrots and many people on BYC have stories about their favorite sweetest boy chick that used to roost on shoulder becoming aggressive.
2 human aggression can Be inherited, always pay attention to your birds temperaments aka behaviors if breeding them to avoid breeding unwanted behaviors in your flock... it is absolutely amazing how many behaviors are breedable for... gamefowl breeders know this.
3 humans can trigger aggression by not understanding chickens or treating them in ways we would a dog but to a chicken you might be the aggressive one... study big bird behaviors as to what they like or dislike... oddly again parrots may help you learn to understand chickens better, especially videos on body language and gentle training.
4 learn to know warning signs of aggressive rooster aka man killers, see YouTube a gamefowl guy put a very informative video up on just that.
5 learn to trim nails and spurs for your safety and the hens safety.
6 don't let aggression slide, let them know who is boss. I peck back, put birds in time out, use the word No when dealing with attitude... there are some threads on correcting bad dog roosters so they need not end up as chicken dinner... some aggression is correctable.
7 know if you are dealing with a broody rooster... defending hen and offspring. Sometimes Roos not just hens get aggressive to protect eggs or babies too.
I hope this helps, most roosters are usually not a problem.