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.
His victims rightfully could have exercised their right of self defense (and perhaps did, I'm not following the story); but it's a far cry between someone engaging in self-defense and suggesting granting a pass for extra-legal, possibly violent, action against another... no matter how reprehensible the target.
I hope you're not actually making a call to extra-legal or violent action and, if so, I hope this community doesn't sanction it.
[edit for clarity]