Sometimes they ARE too aggressive. If you have 10 roosters who mostly get along with each other, and one who relentlessly flogs the others whenever he can, you have to make that aggressive one stop. To take him out and keep him in a pen on his own would be cruel, they're social. Removing and reintroducing him might help. If it doesn't, if you have a rooster who's too aggressive to live in a bachelor flock no matter what, the only humane thing to do is cull it.
It is the natural behavior of a rooster to be somewhat aggressive, yes. That doesn't mean you should allow it. It's inhumane to let roosters continue to fight each other and stress each other out, no matter how natural that behavior is. You have to find a way to make them stop, and sometimes they won't stop. Some roosters are just very aggressive.
If you have 11 roosters and 11 hens, you should only put 1 rooster with those 11 hens. You therefore have to figure out what to do with those 10 other roosters. You could get 100 hens, yes, but that means that someone else has to figure out what to do with the corresponding 100 roosters. You could also try putting those 10 roosters in a bachelor flock. If that doesn't work, or if you don't have the space or inclination to have a bachelor flock, the humane thing to do is cull them. Not leave them to fend for themselves, not leave them to perform their natural behaviors and stress your hens to death. Cull them.