Billy Butcher has a lot of ups and downs in the comics. He has to deal with personal demons and external threats, which leads to some unexpected developments.
In the comics, Billy Butcher doesn't possess powers. He relies on his combat skills, experience, and a never-back-down attitude to fight against those with supernatural abilities.
Some recommended novels where the antagonist becomes the antagonist "are Salted Fish Young Master dressed as the villain's white moonlight" and "Salted Fish Junior Sister forced to become the villain's white moonlight". These two novels depict the struggles and growth of the main characters as they traverse into the roles of antagonists.
One great 'billy butcher fanfic' could be the one where he teams up with an unexpected ally. It shows a different side of his character, exploring his ability to form alliances beyond what we see in the original show. It's full of action and great character development.
First, study the character of Billy Butcher well. Watch the source material closely to understand his mannerisms, his speech patterns, and his values. Then, come up with an interesting plot idea. It could be something related to his revenge arc or his relationships with other characters.