First off, I want to say that I don't have a problem with the concept of character death. Sometimes, people die. Sometimes, yes, even in fiction. And as an author, I don't think I'm allowed to have a problem with character death. There's a point in Stephen King's Misery where Annie Wilkes, the crazy nurse, is accusing Paul Sheldon of being a murderer because his main character, Misery, has died in childbirth. He protests, saying very firmly that he didn't kill her. She just died.
Sometimes characters just die.
I've had that happen to me twice now: characters I really expected were going to make it through their stories have turned around and said "no, I'm sorry, this is where I get off the train." Once that point was reached, I couldn't go back. I walked away from a book for six months once, and I still couldn't take it back. I'm not saying that fictional people have free will, but I am saying that a well-made character will do things the author doesn't expect, and that a story acquires a narrative weight that can sometimes make certain things inevitable. Sometimes the only way an author can have control is to be untrue to the story, and readers can tell when that happens. A good story is alive. Saving a character who's supposed to die can kill it. I may love zombies, but that doesn't mean I want to turn my books into zombies, y'know?
Similarly, if not the same, sometimes you just need to kill people. It's entirely unrealistic to, say, write a zombie techno-thriller in which absolutely everybody lives. So sometimes, I have characters who are just, in the immortal words of Spider Jerusalem, here to go. Not all of them actually leave! Sometimes people I created as "fire and forget" wind up sticking around, refusing to die, while characters I expected to be working with for years politely take their leave six chapters in. I try to be true to the story. I try not to fight it.
At the same time, very little pisses me off more than a bad character death. One of my favorite television shows recently killed off one of my favorite characters in a manner that was unnecessary and just plain mean. It felt like they were going "how can we make it clear that things are getting really, really serious? Hey! Let's bring back this minor character that everyone thinks we've forgotten about, and just kill the crap out of 'em! That'll make everybody sit up and take notice!" Shock and awe deaths don't do anything but upset me. I've stopped watching shows entirely for pulling that sort of stunt—after the episode of Torchwood where I spent an hour crying and saying "I am not okay with this" over and over, I took the show off my watch-list. I dropped Sanctuary over a death that felt less plot-serving and more "the focus group says..."-serving. And yes, there are books that I've thrown aside in disgust, because it all just got to be too damn mean and purposeless to take.
This is not me saying "if you kill a character I like, you, too, are dead to me." For example, one of my favorite movies is The Fly. Yes, with Jeff Goldblum. For those of you who don't know it, it's a horror movie, and things don't go well for most of the main characters. I've been known to watch it when I'm not feeling well, in the hopes that I'll fall asleep and it'll get a different ending in my dreams, Just This Once. At the same time, the ending is so right, and so justified by what came before it, that I don't mind. And that's sort of the thing. When a character's death is right and true and meant to happen, it shows, and those deaths, even when they upset me, are the way things ought to be.
Jim also makes the point—and it's a good one—that killing is contextual. If I kill someone in a Toby book, that's expected. If I kill someone in one of Mira Grant's books, that's practically a legal requirement. But if I kill someone in a Corey book, people are going to be going "Um, w-t-f, over?" and threatening me with sticks. Genre determines a lot of what you can get away with, and what I'm willing to accept as a reader or viewer. I don't like to be blindsided; I don't think anybody does. (This isn't me saying "no deaths in YA," by the way. People will die in the Clady books. Just that the genre really does determine what is and is not okay.)
I will always kill characters. I can't help it. Sometimes the story needs people I care about to die, and sometimes individual stories are just done. I will also always get upset over senseless character deaths, because there's a big, big difference between "this needed to happen" and "I'm the author, that's why."