I think they want to believe it's bad, but don't actually. This is why they do it a lot and keep putting that bit at the end of movies where the hero says "if I kill you, I will be just like you", and then turn his back, whereupon the villain attempts to strike the hero down, and then the hero gets to, in self defence, justifiably get their revenge.

I think it comes down the circumstances of both the inciting incident and what's going on when revenge is sought. Should be proportional at most, and depends on if you have power for the foreseeable future etc etc. Does the initial party want to continue hostilities, have contrition, restitution etc.