When you kill off a character will largely depend on what you want to achieve from their death (or even what you don’t want your characters to achieve!). For example, killing off a character in the first chapter will immediately set the tone of your novel. Killing off a well-known or loved character near the end will be the most tragic type of death.
Always keep in mind that deaths should be written for the characters, not characters for deaths. Every character should have a function to the plot prior to their demise, and they should be characterised thoroughly - particularly regarding their relationship with others. For this reason, I like to wait until the moment of demise (or shortly before) before deciding which character to kill off. There are a few reasons for this:
It forces you to consider a wider range of possibilities for the direction of your plot. When deciding who to kill, you’ll be considering what direction the plot could go after the fact, including the impact on other characters, as well as the plot.
Leading on from the point above, it forces you to consider which characters are important and helps you see which ones are actually useless. If you find a useless character, you’ll know because their death won’t change anything. At that point, you can either make changes to increase their influence or remove the character.
It encourages you to characterise each character to their fullest and deepest extent. Knowing that we’re going to kill off a character can lead us to subconsciously avoid giving a character much depth or letting ourselves or the reader grow attached to them. This will not only lead to a weak character, but will probably also negatively influence the death scene.
It was felt to be a possibility that the hero would die. That’s what I was aiming for, that you really felt that anyone was up for grabs - J K Rowling (2007)
Take J K Rowling, for example. Rowling has made it known that she didn’t have plans for which characters she would kill during the Harry Potter series, and especially during Deathly Hallows, she flirted with the idea of killing off all sorts of characters - but definitely didn’t go through with all of them.
She is quoted to have said:
Funnily enough, I planned from the start that none of the trio would die. Then midway through, which I think is a reflection of the fact that I wasn’t in a very happy place, I started thinking I might polish one of them off. Out of sheer spite. “There, now you definitely can’t have him anymore.” But I think in my absolute heart of heart of hearts, although I did seriously consider killing Ron, [I wouldn’t have done it].
So with that in mind, let’s get started!
Unfinished / Finished Business
Give your characters unfinished business at the time of their death, or you might kill them immediately after their business is finished. They may die while trying to complete their goal, or alternatively, complete their goal but die in the process.
Dobby's death in Harry Potter and the Deathly Hallows is a good example of 'finished business', where Dobby's final act was to repay Harry for setting him free in Book 2 - except, he repaid him with his life.
When a character sacrifices themself to save others, or is sacrificed to save others, the nobility or unfairness of the act can make it all the more sad. Especially if, earlier in the plot, the character on the chopping block has already been through a lot, or has saved the people they are sacrificing themself for once already.
Take Old Yeller from Fred Gipson's Old Yeller. Prior to being unfortunately shot due to a rabies infection, Old Yeller saved his family from a bear, and then a wolf.
It came clear to me then that Mama was right. We couldn’t take the risk. And from everything I had heard, I knew that there was very little chance of Old Yeller’s escaping the sickness. It was going to kill something inside me to do it, but I knew then that I had to shoot my big yeller