Reasons Why They Can Be So Mean I have saw numerous cases of aggressive roosters.Part of the reason their so aggressive,is sometimes,it's genes,or their hormones.If a rooster let's say you bred,has had mean parents,or if the parents of them were aggressive,it all can direct back to that chick.For instant,gamefowl birds.Most times they are used for fighting,it's in their blood,and genes carry down the blood line.Which causes those roosters to be aggressive.
But when it comes to hormones,there are not very many things you can do,but try do discourage the behaviour.What you can do is,stand your ground,never dart off,that shows weakness,and the rooster thinks he has got you.Turn around,and hold the bird,or run at him.Sometimes,not even this work,it's hormones,it goes against them,they cannot help it.
Getting and Treating the way they need to be Treated
If you do have plains in the future to get a rooster,I suggest by a little chick to start off with.I don't like buying adults.The reason for this is because,you don't always know what the roosters being fed,or why their giving or selling it away,but that's just me.You should handle roosters from babies,up to adults,this teaches the rooster that you will be holding him,and his girls (hold the hens very often).Also,never walk around a rooster,walk straight threw him,if you walk around it shows them respect.Do not push him with your foot if he doesn't move just keeping walking,he will have to move.
(This is the the "Dominant wing or known as the Rooster Dance" regardless of what it is,it shows dominance,and when it is done to the hens,it is to show dominance and to impress them.)
Some ways they can show aggression is the "Chicken Dance".Discourage,it can escalate really quick.I have saw roosters do this to other roosters then circle around ans reach the face and begin lunging,spurring,and pecking,they will do the same to us.
You can also castrate the roosters.This makes them calm,stops fight,stops crowing,and from them potently being aggressive.This is called caponing.Thanks to QueenMisha,I know a lot about caponing.