The advice to ‘kill your darlings’ or ‘kill your babies’ is dispensed so often in writing circles that it ranks among the most over-used writing clichés (right up there with ‘show, don’t tell’).
What it means is that you need to be ruthless in your willingness to cut sections of your writing that don’t work to strengthen it — even if it’s some of your favorite stuff. If it doesn’t serve the piece overall, it’s got to go.
But I like to take this advice in a second way: as a directive to treat your favorite characters just as ruthlessly as your favorite sentences. You need to put them through Hell, run them through the ringer, and — if it serves the piece as a whole — kill ‘em too.
If you’re familiar with George R. R. Martin’s Game of Thrones, you know he’s a bit of an expert in this principle (no spoilers here, so don’t worry if you haven’t gotten into them yet). All you need to know is that Martin has a penchant for making you really like a character, to really care about a character, then killing that character off as if they were nothing more than an extra (or at least a stupid frat boy in some horror movie: “Helloooo? Who left this chainsaw running? Hellooooo?!”)
Mind you, I’m not saying that you should kill your characters when it makes no sense. If you’re writing a romance set in the 18th century, an asteroid probably does not need to crush Mrs. Haberdashery’s tea party. Likewise, it’s probably best if your main character is not eaten by a dingo on the first page.
What I am saying is that, if it’s realistic in your story world that one of your characters could be killed, you should seriously consider whether it might make your story better to let it happen. Even if it’s the best character you’ve ever written, and it nearly kills you to do it, you need to be able to recognize when it serves the piece as a whole just to let them go.
This concept can also be applied to hardship in general. Your character’s life doesn’t need to be easy, and in many cases it shouldn’t be. One problem I often run into is that there isn’t enough conflict in my stories, that I’m not creating enough obstacles for my characters to overcome. So, any time there’s something that could go wrong for your character (maybe his car breaks down or his tooth breaks on a hard croissant, or his Aunt Haberdashery gets crushed by a space rock), you should seriously consider letting it happen.
Sometimes it’s more realistic to let things go wrong than to let them work out right, and it always pays to put your characters under severe pressure. That’s when they show who they are; that’s when they make the hard decisions that really drive the story.
So, kill your darlings — or at the very least make them wish they were dead.