If it is dark.... the chickens are all roosting, sleeping.. you can go in and quietly pick one up when they are asleep, roosting. Take a towel to wrap him in and have a cage to put him in ready to go... supplied and open. (or some other place to isolate and contain him until time for the "hangman")
I know that I don't have the heart to kill my own chickens, but I have a neighbor/friend who will take care of any "problem" I have... soup, BBQ, or trash. I am grateful for someone else to do the dirty work for me when needed.
An aggressive rooster can put out an eye.... my brother had one that was aggressive, it came at him flying up.. cut his cheek (with its talon)... my brother is a big man, 6'3"... think what it could do to a child, or you.
"Raising Chickens for Dummies" says that the very first time a rooster comes after a person: SOUP. No exceptions, they only get worse.
Again, in my opinion, the secret to catching him, containing him is to get him now, tonight, while he is roosting, sleeping. You might want to have someone to help you, another adult to hold the towel and be ready to help hold him around the wings.
I'm sorry you have to go through this. Good luck.