There is no humane way to kill an animal. He isn't being mean, he is probably just being a good rooster and trying to protect his hens. Why not bring him to your local animal shelter?
So what do you do if one of your chickens gets attacked by a predator and is mortally wounded? Or if your dog or cat comes down with some disease or illness and is suffering, no quality of life anymore, whatever, and should be put down? Do you just let them suffer? Keep them alive for your own sake because you can’t bear to lose them?
I don’t mean to be mean and my chickens are my pets so I could never kill mine either, half of mine are 4 years old and they'll stay till the end of their life, so I get not wanting to kill your animals or having chickens as pets or whatever, not everyone will but I totally get it, BUT on the other hand, I think it’s a little bit ridiculous to say that there is never a humane way or reason to kill an animal. And I also think it’s quite selfish and irresponsible to let an animal suffer or to dump your problems off on someone else because we don’t to deal with it. If my animal was in pain and suffering, even if it was my pet, I would put it down.
And I would never take a mean rooster to a shelter. Especially since most also charge to drop animals off and/or won’t let people kill the animal after adopting, which, if it’s a dangerous bird is irresponsible. Although that said, I MAY consider putting the bird up for free on Craigslist or something but ONLY if I fully disclosed that he was a mean bird and would be best for a meal. That way someone else can at least take him and get a meal from him.
But I still think it is best to just deal with it yourself rather than dumping the problem on someone else.