I have always admired authors like George R.R. Martin who take characters to their maximum extreme of torture and only when they peak over toward the side of victory does he slay them to pieces. It's truly remarkable that you can kill off some of the most likable and loveable characters and can still manage to grip the readers. Perhaps pulling a G.R.R.M. isn't the correct route all the time though. I feel he has so many characters that keeping them all in the main story line is just too much to keep track of. In order to find semblance and order within the whole arc of the story you would have to kill off characters, but when is the killing enough? Not everyone can win, and for the most part the evil ones win far more often than the good ones. Once again....poor Sean Bean! But the more I see/read the more I can't help but think, "Oh no, don't kill that person!! I really really like that character!!" But to that I say, "LIKE ONLY THE ONES YOU HATE, because George will kill off every likable character every time you ask him when his next book is coming out."
Diana Gabaldon pulls her characters to the highest of highs before ripping them back down to base zero stripping them of everything they know. She has a way of making the characters struggle toward happiness and survival, and she succeeds in making you believe the anguish and horror they experience. I think of most authors Diana has found a great way to lead you into loving a character and hoping along beside them that things will work out. She also takes you on wild roller coaster of a story making you want to read through a boring chapter or two just to see the characters on a safe and relaxing buoy in the storm of crazy problems. Diana doesn't let you down though, and never did I expect a character to live that she had killed off neither did I necessarily grief at their passing. Each and every kill I feel is expected, earned, or deserved. And if there is one thing I can say for certain you know she won't kill off Jamie or Claire, she just won't because it would destroy her readers ( I haven't even read all the books and I know this!!)
Then there are authors like Veronica Roth who have a seriously great story line going and then ruin everything she built up by enacting a completely horrible death. She pulled in readers with the idea of her story, the whole fighting back against social structure and dominating leaders but the last leg of her trilogy fell to pieces for the readers when she killed off the main character. There are few times when I shake my head at authors decisions, few times when I disagree with the direction they took with the story line, but in some cases I just can't get on board. Why does it work for some authors and not for others?
I think some could argue that you don't really need to kill off any characters. Go for a Lord of the Rings style story and pretend to kill off super important characters only to have them fight an evil shadow beast on top of a mountain and come back as a white cloaked God like figure. Now one could argue that Boromir dies (Poor poor Sean Bean, you can never catch a break can you!) but to that I say, "Of course he dies, he is tainted by the ring and knows he can't deny it's power." Boromir accepts his path toward denying evil, in order to truly make things right with Frodo he must sacrifice himself for the remaining hobbits. In books like LOTR you don't see the author killing off likable characters, you see him killing off the characters that are either evil or accept their evil ways and make a change for the better through sacrifice. I think both of those are extremely acceptable.
So the question stands, to kill or not to kill, and to be honest even in the middle of a scene with sword/gun/ knife/ bomb/ thumbs poised to inflict the killing shot I still never know if the character is going to remain alive or not.