For writers, I always think that we see our characters in two ways, for their instrumental and intrinsic uses. This simply means that we create them as a means to an end, whilst at the same time give them enough individuality and self-importance so that they don't seem to melt laconically off page after page. Readers probably struggle more to see the instrumental use, and after all, that's not their role. Writers are the ones who see a book as a string of events, dialogue passages leading to another, doing something to accomplish a certain action which is necessary for the story to continue and, naturally, get better.
So when that action is killing off a character, as said before, there has to be a real reason. You have to consider the possibility that when you kill off a character, the readers aren't going to understand it. If they fully did, they'd probably be doing your job for you a little bit, and I always think it's important to maintain that barrier, even if readers think they know all the secrets and that you'll accept all of their suggestions. We're writers, we're that eccentric collective of people who point out that humanity's obsession with immortality doesn't have to be solved by experimenting on bacteria or cryogenic freezing.
Non omnis moriar.
Anyway, I digress. So writers know the values of their characters in both senses, of what they bring to the story and what they bring to the story - how they enrich it, not simply how they allow for loopholes or rambling explanations about something that another character couldn't get away with. So, when the story defeats the purpose of that character, hard decisions need to be made. It's difficult because we make emotional investments in our characters (sometimes literally blood, sweat, and most of all, tears) and the temptation is always there to keep a character in the story because: you love them intrinsically or, even worse, because your readers love them. It was difficult for me to remain tight-lipped when my best friend (who has been reading Call to the King for about a year through all its changes) rambled on to me about a character that she loved, who she felt was most like her, when I sat there knowing that in two books time that character was going to meet a tragic end.
But as much as I'd like to please her, and as much as I know she'll despise me for a week when it comes down to it, I can't ignore my instincts. Its this rare sensation I get when writing that I can't ignore, when I'm sitting and thinking of many different scenarios in my head, and one of those is a character death. It's unexpected, I didn't want it to happen, but the scene comes together so perfectly in my head, the reacting emotions are so real and so powerful, that no matter how much I try to brush past it and think of another solution, at the end of the night I have to curl up before bed and say quietly to myself, with complete resolution, "they have to die."
I always think about Greek literature when I think about characterisation, because those like Homer and, much later, Virgil took characters as symbols to a whole new level. What a character's role is in a household or in society directly relates to their effect on a hero, with which Greek epic was obsessed by. Previous to Book 1 of The Aeneid, our resident hero Aeneas' father Anchises died on Crete. Did Virgil do this to make Aeneas suffer, or because Anchises was a boring old character who had no place? Not really, but what the character death does is take Anchises, the father figure, transmute him into simple concepts, and eradicate them. Aeneas flees Troy with some of his family, having lost his wife in the chaos (first strike) and now loses his father (strike two.) Aeneas carried his father on his back in Book 2, a flashback of Troy, and Anchises, representing the remnants of his son's old life, weighs him down. So to truly begin another life, Aeneas must be left bare of all ties to Troy, apart from his son, as the axis of the story is that Aeneas' descendants will found glorious Rome.
I hope nobody thinks of this as me rambling about what I wrote in my Classics A-Level exam. What I'm trying to demonstrate is that a character does have a specific purpose, and sometimes we as writers don't always recognise that ourselves. Did I make a list of all the different symbols I needed in my story and then write characters matching that? No, but when I realised what they represented, it only enriched how I wrote about them. So even if your readers will hate you, and argue against the decision, unable to realise the reason behind it (at least not in the short term), then I'm saying that's fine. As I said, if they knew the ins and outs of your head, you wouldn't have your secrets, and then I don't think you'd really feel like a writer.