I think you have to find the line where revenge can do more harm than good, and where revenge is just going to end the arc (so to speak)

In Montoya's case it brings the killing to an end, because The Count (as far as we know) has no one to miss him, no one to seek vengeance for him and no one to get pissed that he is dead. So when Montoya stabs him that's it -- The Count is dead, Montoya's need for vengeance is satisfied and the story is done.

But if you go to something like Romeo and Juliet (or West Side Story because I can remember the character names), then getting revenge there just causes the cycle of violence to continue.

Bernardo stabs Riff. Tony stabs Bernardo. Chico goes looking for Tony, and eventually shoots him. And then -- at the end, the two gangs want revenge.

Now clearly if Maria kills Chico for him shooting Tony, it won't bring Tony back, but it will cause more anger in The Sharks.

And if The Jets kill Chico for shooting Tony, then it won't bring Tony back, but it will cause more anger in The Sharks.

And if The Sharks get revenge for Bernardo's death by taking out Ice, then it won't bring back Bernardo, but...... you get the idea.

Which is why Maria says "no", and tells them to let it go. She might loathe Chico and want him to pay for Tony's death, but killing him won't help.

I grew up with West Side Story and Xena, so I am more on the "don't kill people for revenge" side of things.

Not that I am saying Montoya was in the wrong, of course :)