You can't accurately judge anyone's actions out of context. If you shoot someone, that doesn't sound good. But if you shoot someone to save your live, suddenly you're not judged as a bad person.
Tybalt's blackmailing is selfish, pure and simple, so it's an evil act (especially if perpetrated against an innocent virgin!). But if the MC threatened Tybalt with watermelon fucking photos to get him off Jill's back, would that be an evil act? Both involve blackmail. And both have the same goal in mind (both guys want to fuck Jill).
I mean, if you shoot someone in self defense, then I would argue the key part here is that it was self defense (assuming you used justifiable amounts of force). If that's the case, then I don't really care about the details of how you did it.
And I sure can judge without all the context! It's all based my values and whether or not something goes against them, but
after said judgement has been cast you can add in extenuating circumstances. I don't want to be killed, you don't want to be killed, damn near everyone does
not want to be killed, so I conclude that it is immoral to force death upon others (based on a load of other values), and self defense is still a very strong case for extenuating circumstances. And there's a difference between judging an act moral (or not) and ethically justifiable (or not).
I mean, in a comment like half an hour ago I acknowledged that you can do bad things for good reasons. I think killing is bad, but I think self defense is a good reason. Using your example, I think counter-blackmailing Tybalt to neutralize the situation because it's the right thing to do is morally fine, maybe even a sign of truly exceptional moral fiber for going out of your way to protect someone like that. I mean,
I probably wouldn't do that because I would still feel like shit and probably anxious, but I also wouldn't fault someone who did take those steps in that situation. Mostly I would worry about them getting into trouble. So, not moral, but probably justifiable.
But (counter-)blackmailing someone just because
you want to wet your dick? Now you're no better than Tybalt, and you are both in the wrong. That's not blackmailing to right a wrong, that's blackmailing to gain an edge on your competition.
And, uhh, I guess I better wrap up, I tend to get a bit carried away with these subjects. I guess I made my case, but I want to add that intent matters a lot when discussing morals.