The definition you quoted was pointing to the film genre itself not the antagonist character themselves, I was laughing at the incompetence of you quoting such a definition and thinking it made your point.
There are definitions that do make your point, you just didn't post one, but ultimately it is a definition of an english word, not looking at the original Japanese and looking at the definition of that word.
Oden slashed the white boar, but that thing is still up and around.
"Slashers" irl would kill their targets, in OP world where people can survive nukes and being beaten with bedrock busting fists with a few days rest, not so much, so context is missing. Trying to use common parlence as indisputible proof that "Zoro kills" when he fails to kill all named serious enemies makes you look like a joke.