Yes vigilantism is an affront to the rule of law. But if the rule of law demonstrably fails then it is necessary. (It shouldn’t fail but that’s a different matter.)

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.