Eh...if the vigilantes used discretion and only killed big names (like the Joker) I doubt the public would be any more or less tolerant of them in general. On the contrary, I think it may play into the public's own desire for retribution and revenge, which may not be a good thing but still clearly exists. Superheroes go above and beyond the law all the time, some people may be quite content to see them take that last step and simply eliminate criminals who repeatedly elude justice. Maybe.