A lot of action-adventure tales I feel cheated at the end because everyone survives. Or at least everyone who means anything to the reader. All that danger and it couldn't actually kill anyone? (Forget the faceless masses and the red-shirts, who were clearly created to die; where they are concerned we are quite aware that they are really black marks on white paper and not real people, however subconsciously.)
I don't like the deaths when they happen -- deaths of real characters, not furniture that happens to be humaform and talk -- but they do produce emotional reaction. And underscore that victory is not costless. And make the danger convincing. ( I was reading a complaint about Hunger Games where a (hopefully young) reader was complaining that what he had wanted was for Katniss to figure out a way to save Rue. Ah, no. Death in Hunger Games is not reserved for characters you don't give a damn about; that was due warning.)
Though there were deaths that did not convince me because they happened too quickly. No matter how quickly the death occurred, death is an important matter and needs to be dwelt upon. In the POV character's thoughts, if nowhere else.
Then there's improbable survival. For that I have come to the conclusion that the more frankly supernatural your means are, the better. I read a comic once in which some Norse gods found themselves in the land of the dead, facing a character killed in the first book. So they dragged him back to life. I don't think anyone has any right to object to that. Where it goes astray is when vastly improbably but putatively mundane techniques are used to explain away a death.
If a death has to be shown to be false, I find it best to wrap that silver lining in a very thick, very dark -- stygian even -- cloud. Jack survived the explosion, but the villain is using him as a hostage. Jill survived holding the line against the hordes of goblins, but was deranged by her sufferings and remembers only that you left her to go on, and not that she ordered you to do so.
Other genres have other rules. There have been stories where I reacted to the death of a character as cheating. The goofy comic relief fool of a character? He's not supposed to die! You haven't set the "grim and gritty" setting high enough to make it fit! It can be a real wrestle, getting into the early chapters that this is a universe where people die. Even people who have Plot Armor (which makes plate look wimpy). For one thing, you have to do it without escalating the tension so dramatically that it's hard to go up.
Some real deaths are awfully convenient. The villain, repentent, dies stopping them not because he is seeking death but because the writer doesn't want to figure out how to get him off or to send him to jail.
And then there's where the Grim Reaper himself. Or a psychopomp (as in Gunnerkriegg Court) if you don't want him to actually kill someone. It can change the whole tone of dying, that way.
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