I think you always need to keep your readers in mind when you kill off a character. How are they going to feel when you do, and—more importantly—how are they going to feel about the rest of your book? If you kill off their favourite character, especially if you do it for no good reason, it will be difficult to get them back on board for the rest of the story. A death of a main character will impact the story. (Think Eddard's death in Game of Thrones.) The death of a minor character doesn't necessarily upset a reader too much, but at the same time it's not going to impact much either. The trick there is to have the minor character's death mean a LOT to one of your main characters. Then you can get the death working for you in your story, even if the reader isn't horrified or saddened by it. Too many deaths, like too much violence, too many hand-to-hand fights, too much sex, too much of anything, will get boring to read, and your readers will switch off before the end. Make your characters' deaths count.
I think when you asked "How would this advance the story", you struck on something important. Unless writing a slasher novel, typically a character is killed off when it advances the plot, or is necessary to make the story believable. For example when a character gets killed, it could give another character a motive for revenge. Or maybe you have a lot of dangerous fight scenes in your book, and it wouldn't seem realistic if every character came out unscathed.
You shouldn't be killing off characters just for the sake of a death. The death needs to make sense in terms of the story and actually bring something to it. I was watched How to Train Your Dragon 2 the other day (SPOLER ALERT!!!!). I guessed the plot twits with his father as soon as he met the mother again. I told my friend and when he did die she asked me how I had managed to guess. I simply responded that had I written the film I would have done the same as his death served a purpose of spurring the main character into the role he had always meant to fulfill. It made total sense and that is why it was alarmingly obvious. A character death can be as a result of stupidity and serves as a warning, or it can be a tragic accident that spurs the protagonist on. It could even be as a mindless act of violence in order to reinforce the inherent evil in the antagonist (or even push them towards evil). If the character is meant to die, the story will tell you.
When they as a character are no longer relevant to the story or when their death is relevant to the story. I'll include various forms of exiling in the killing off category, except that they can zombie back into the story later if they are merely exiled.
While I am guilty of killing at least one character in any story, (I am not a fantasy/science fiction writer for the most part, a multitude of my stories are reality-based) I think killing a character can become a real drag when it is done merely for the shock value. When you write in a character simply because he/she is going to die and make your MC push the plot forward, it can be tad bit weak and clichéd. Especially if the character was underdeveloped. If you feel the presence of your character, dead or alive, doesn't impact the plot, he/she shouldn't even be in the story, obviously. As for whether or not to kill a character, I think that death is the significant thing to happen in any person's life, especially your MC, and it should represent your theme, your main idea. Like @Joe Nathan said. For instance, in a story of mine, a young man kills his abusive, deluded father, and nearly ten years later he kills (by proxy) his cousin, a lovable and amicable person who has always wished for the better of others. These deaths represent how he had become the same monster he had killed, how unwittingly he was his father. That is the theme of the story, one of them anyway--hatred and revenge can turn people into the very things they hate. I'm no expert but while deaths in a realistic novel are rare, (unless it is preset in war or something) these deaths seemed purposeful to me, and so I don't feel silly about going through with them. You may want to make the same judgement call.
The problem is, when you kill off characters whether they're good or bad, minor or major... you've got to make sure you know how you write it and give impact to the people surrounding the deceased. If the antagonist killed a random person in front of the protagonist, it doesn't affect them so much. But if it was someone important, then its going to hit the protagonist hard. This could go different ways, one example from my story would be vengeance - most of my characters have adapted to vengeance because it's one of the curses. If it's not vengeance than it's self defence, another would be that the character strives to help out anyone before the death can occur again.
Just because characters are killed off for shock value, does not mean that is a bad thing. The point of art is to evoke emotion from the viewer (or reader in this case). There is never a perfect time to kill off a character but it is done best when least expected. I really enjoy it when a character's death is sudden and unexpected. If the main protagonist is a flawless hero then they should definitely be killed off in the end. If the reader sympathizes with the protagonist in any way then it is important to kill them off.
That's only worth anything if you then deal with the aftermath - no character who knew the deceased should go unaffected. But if you simply kill someone off "just for fun", as it were, then it is a good story-telling device gone to waste. Death is significant and can be the difference between a memorable story and simply a good story, but used wrongly and you risk enraging all your readers and giving your readers an unsatisfying ending. And if you have dissatified readers who feel the death of their beloved character was unjustified or unreasonable, then no one's ever gonna read your work again. I imagine Game of Thrones author (George something Martin? I can never remember) isn't successful because he kills off his characters constantly and unexpectedly - his success lies elsewhere, and I imagine if he wasn't good at other aspects, the frustration of constantly seeing your beloved characters die would've sunk his work a long time ago. I don't think it's good habit to kill characters off simply because your reader sympathises with them. By that definition, you'd have to kill EVERYONE in your book except the villains, and then perhaps even them if you happened to write especially complex and therefore sympathetic villains! One would imagine a skilled author should be able to make all his character sympathetic, after all. The occasional death for shock value is probably okay. But you do it in every book or to most characters and it becomes OTT. Death in and of itself isn't shocking if it happens constantly - just annoying.
Well I don't think annoying or unsettling your readers is necessarily a bad thing. If an author chooses to leave their protagonist alive, it must be justified. Death does not have to be justified. A story must finish with death or it must not finish. In life, death is the end of our short-term collection of energy. If the book does not end with death, then it has not ended. That goes for any work of art. That is why dogme 95 films are so beautifully accurate. Leaving an ambiguous ending by simply cutting the story half way through is brilliant. An ending does not have to be satisfying. It is not the author's job to entertain people. It is the authors job to express themselves by creating a work of art. I do agree with you on many points but specifically this one point. A good author should be able to create sympathy for all characters as this would be an accurate portrayal of the human condition.
Sorry but I have to disagree here. Maybe I'm old fashioned but "and they lived happily ever after" is a fine ending. The action has come to an end, somebody (the good guys in this case) has won, and what they do after is not relevant to the story. I'm planning to pair off a lot of my characters romantically by the end, and I'm sure they're all going to have interesting marriages, careers, make lots of kids, whatever...but it's not entirely relevant to the story, and if I ever return to them after reaching that point, then it will have to be a new conflict. I'm not particularly interested in what happens after happily-ever-after, so I end it at that point, and it's satisfying. Death is part of life, but life is not about death, and a life can be fulfilling and satisfying even if it doesn't end with a heroic or tragic death. I like my characters, and like any real people I like, I hope they eventually die of old age, comfortably, in their sleep, after long and healthy lives. And unlike real people I'm actually in position to ensure that happens, so that's what I plan on doing. And that doesn't mean nobody dies in the story - quite a number of people do in my case - but my characters are survivors, and they survive.
If that's your take on fiction, perhaps you're more suited to write literary fiction. Nothing wrong with that at all, but there are different genres, and different purposes an author wishes to achieve with that. For that matter, people also read for a variety of different reasons. Entertainment is, without a doubt, one of the reasons. If you are adamant that you will NOT focus on that and/or write for that audience, that is fine. You will simply write for a small, niche market. But I think that's kinda cool too, you know? That there are passionate writers who care and love these different aspects of fiction and therefore cater to these many different needs of the readers. For my part, however, to refuse to put any effort in entertainment is, I think, a mistake. Simply because when a reader enjoys a story, they usually remember it more. They usually think about it more. When I say "enjoy" - I do not mean a happy ending or ones without death or thought or tragedy, but simply a story that was fulfilling for the reader. Truth is, if the reader does not feel fulfilled at the end - and the wrong ending can very much do that despite an otherwise satisfying book - then the reader will not come back for your work. I can reach a larger audience if I give some room to entertainment - if I seek to have my readers enjoy my story. For example, I thoroughly enjoyed I Am Legend (the book) and the ending was by no means a happy one - and I enjoyed it. It not only had a satisfying ending, but it made me think as well as thrilling me as entertainment. And that leaves a mark and it makes me ponder its message. On the other hand, give me a book with a great message but that which is not entertaining at all - I'm sorry, but I am a fairly impatient reader - I'd never finish the book. For that reason, I never made it past p.50 of Crime and Punishment. And then there are other times when I do just want a laugh and to relax. Those books are mindless and maybe even badly written from an artistic point of view, but I adore them, because they gave me relief and joy at a moment when I did not have them. They become special to me for that reason. And isn't that worth something too? So while I agree with you to an extent, I do think that might be a bit of a narrow view of the purpose of fiction. What you have stated is but ONE purpose of fiction, ONE way of writing - and it is a good way indeed. But it's good to remember there are other purposes, other ways too. Anyway, with your way of focusing on using fiction - you might be very well placed to write about suffering. If death does not have to be justified - then what you're really touching on, seems to me anyway, is the pointlessness of death and/or suffering.
People die, that's the way things work, and almost never In ceremonious well-fought ways. in both stories I'm working on a main character is killed off at the end abruptly and without much fanfare, and not for shock value (though the way they are achieved could be fairly shocking, though in one case not entirely unexpected ) but because it's a major development role for the characters involved, and in the start of part 2 for both, the plot begins by showing the consequences of the other characters actions. Don't have a character whose sole purpose is to be killed, and don't kill them off without a plan. Death may be unceremonious, but it's consequences can be huge, and how characters deal with it can be a major point of character development, rather than just something to shock the readers.
