There is still an inherent negative aspect to the "Don't Fuck with Cats" doxxing. Vigilantes publicly revealing the identity of (suspected) perpetrators can enable further vigilante action, and this can cause harm to innocent people if the identification was incorrect, or unwittingly impede law enforcement. And that's before considering whether vigilantism is inherently good or bad.

See the canonical example of this going wrong: the Reddit 'investigation' of the Boston Bomber, where someone was misidentified, doxxed, and their family was harassed.

Of course, law enforcement is capable of making the same mistakes. But ideally they have better safeguards, and victims of their negligence have much better recourse.