Vigilante justice is an affront to the rule of law and is never justified. Even in heinous crime such as those committed by Weinstein.
Your principle going out the window may be me expressing a deeply held opinion.
Maybe mild preferences.
Not that I don't share a large part of your cynicism.
This is the ethical dilemma, deontology (rule of law) vs utilitarianism (best outcome). It is always easy to construct counter examples to supposedly perfect laws. Don't kill people. Sorry, I'm gonna kill Hitler. Ultimately everyone is a relativist.