There are many movies in which the hero does inflict violence on others in order to succeed. Is there any thin red line when it's okay to be entertained by that and when it's not?
One poster had an interesting point in the Punisher short news thread. He basically said that the violence done by the Punisher should always be felt as negative, because it morally is in reality. Is that the way it should always be in movies? Are we only allowed to find violence amusing under specific rules, when it's somehow justified?
Possible justifications to kill villains:
REVENGE - the villains killed/kidnapped the protagonist or his friends/family and therefore he is allowed to strike back against them and everyone who helped them (Rambo, Death Wish, The Crow, The Punisher, Blade, Kill bill). One of the biggest criticisms of the Thomas Jane version was the fact that his Punisher tried to kill as few as possibe, instead choosing to use fake hydrants. When the Ray Stevenson version had the character literarily punch faces in, it was highly celebrated and the movie wanted you to feel that way. Same for Rambo, Paul Kersey and Blade cleaning out street scum. Fast forward to Law Abiding Citizen, in which charismatic Gerard Butler does the same, only getting even more brutal. He drugs a guy to make him live as long as possible through dozens of amputations. In the latter, we are meant to think he's too extreme, it's clearly seen in his near insane eyes. Now on Punisher War Zone, we are made to want even more kills, more extreme violence. Rambo's first ultra bloody trailer practically screamed "You want blood? You'll get it, and more!" So what's right?
Turn it around and look at Final Destination, Nightmare and the Fridays. These flicks mostly root for the bad guy to murder innocent teens, in various and extreme fashions. What justifies rooting for Jason to bash some girl's head in with her guitar? Because it's absurd and not to be taken seriously?
LAW - in an extreme future, the law representing protagonist is allowed to kill criminals and does so with no remorse (Robocop, Judge Dredd). Only because it's a future vision where violence against criminals is more accepted than today, are we allowed to enjoy it? Do we need to learn first that the thugs in Dredd are raping, killing pieces of shit before it's allowed to be amusing watching Dredd shred them?
FINAL DEFENSE - e.g. the showdown of Mission Impossible 2; the villain seems defeated, only to raise one last gun - and only when that is raised, the hero shoots em (also, every Scream movie). Would it be too dark for Ethan Hunt to just end the villain right there on the spot? He's an agent with the licence to kill, it's not like he has to file an explaining report like Murtaugh and Riggs would have to. In this world, in their's they obviously get away with killing whole armies and attacking diplomats.
STOPPING A THREAT - 24 and Dexter put a heavy focus on that. Jack Bauer kills and tortures tons of people to stop national threats, but there's rarely a moment where he reflects and looks at all the lives he took. He criticizes Annie who becomes exactly like him, but he quickly gives in and basically says everything is necessary for the greater good. If we don't cut this terrorist's dick off and shoot his friends, we will never find the nuke in time. Millions will die. Comes in Dexter, who also saves neighborhoods from murderers and serial killers, but also does it because he's serial killer himself with a thirst for killing. Dexter is charismatic, but the show runners never let you forget that he's a sick man and not a one note comic figure to cheer for.
PASSIVENESS - when Batman kills Ras, he does so by simply not helping him escape the train. That's only a small step into darkness but it has the same statement: find something to justify the villain's death and go for it. Then in TDK, Batman actively saves the Joker from falling to his death. Did he learn and get back on the moral side? Does that scene say that not having helped Ras in BB was wrong?
LAZINESS - when Indy is too bored to have another fist fight, he simply shoots the melee guy on the market (Raiders). What probably is the lightest, most fun killer of all may appear as the most wrong, as Indy openly kills a man in broad daylight just because he's not in the mood to fight him.
In most of those cases, the movies tell you that it's okay for the character to kill if one of the justifications fits. Would any other decision hurt your entertainment? What if Blade called the cops on that vampire club? What if Indiana Jones sued the Nazis? What if Batman killed the Joker in the batpod scene?