I'm one of those wussy readers who can't bear to "see" anyone die. Weird, I know, since I read a ton of murder mysteries. Still, if I'm reading fantasy or some sort of quest story and I follow a character (not the villain. Handled right, I can deal with the villain's well deserved death), feeling their part in the band of heroes, watching them chase their desire, grow and change. Well, if I've been on their journey, for it to end in death makes me wince--and think twice about reading further books by that author.
However, the high stakes fantasy adventures often need a death. It enforces the price of whatever the quest seeks.
So which character can the author kill off, and how can they make it seem right?
I think I'm willing to accept a character who can accept themselves. In a sense, they've reached the point of their journey. Now they need the new challenge, even if it's going to be "off-stage", in death.
But there are also characters whom the author introduces and keeps in the readers' eyes, but doesn't allow to grow. They are one trick ponies the whole way through. Generally decent according to their moral code, but not engaging us in their desire. Being two dimensional, the pain of their death isn't as great. They've moved around the story, but they aren't at the heart of it. Even in death, they remain only a plot enabler for the hero.
Killing off a character who has engaged the reader's emotions without allowing the character to reach the end of their journey might be tragic (I can think of examples where their death highlights a moral point), but I don't like it. I concede other readers aren't so wussy. Still, the crafting of a character the reader will watch die is tough. Thank heaven for the numerous writers out there who respect squeamish readers and keep their characters breathing.