“One Death is a tragedy, one million deaths is a statistic.” Allegedly spoken by Joseph Stalin, so it’s regrettably a Real World Thing.
But it’s also something for writers to keep in mind.
You can have all the galaxy spanning wars you want, kill entire planets full of humans and aliens. But if you want to make your readers really feel it, you have to kill a character they know well enough for it to hurt.
The course of a story plot, the succession of scenes that pulls the reader through the story will involve a lot of ups and downs.
Some writers start with a large down, if you can do a fast sketch of a character before you kill them, you can create a lot of emotion, grief, guilt, anger, in the Main Character that resonates with the reader and pulls them in for the revenge or redemption or acceptance.
But as to the later ups and downs of the plot, traditionally the deepest, darkest, worst down right before the hero picks his ass up, thinks up a new plan/has an epiphany/grits his teeth and buffs up the determination and heads out for the final battle. Depending on the story.
That last deep, dark, hole, if you’re going to kill someone, should be caused by you (you horrible writer, you!) doing the dirty deed, and horrifying the sobbing reader.
It can be the MC’s fault (but like as not he’ll feel responsible even if it isn’t,) it can be entirely accidental (not as much emotional impact,) it can be heroic.
Do not make it unimportant, or worse, boring.
The amount of blood, gore, and screaming will vary, depending on the sort of book you’re writing. If it happens in the middle of battle, the MC’s reaction needs to be short (and probably violent) with his emotional reaction coming later, when he’s got the time for it.
Now I’m not saying that you always kill someone. But if you need to, do it right, and make it drive the story.
And if you’ve never read one of my books, start here: