I agree with @Folly's place. It will just be more difficult the longer you wait to do it. I faced this when I knew it wouldn't work out to have three roosters. I made myself cull when my cockerel was six weeks. This is the method I used. https://the-chicken-chick.com/how-to-humanely-euthanize-chicken-by-dr/ It's very easy and quick and no violence or blood or suffering. The chick didn't suffer, either.