How gory should your story be?
“As gory as it needs to be” is a cop-out. Dracula, one of the most horrifying books ever (I rate by books that kept me sleepless with the lights on), has very little blood in it. And that’s a story about vampires. There’s a lot of hinted blood-drinking going on, and one scene with Mina that reads as a vampiric rape, but most of the book’s violence happens off-screen. The horror element is not gore, but the fear and terror of the world you know being shattered, to reveal unclean abominations lurking in the shadows.
Parasyte the Maxim, on the other hand – this alien invasion has blood all over the place. It could have been made less gory; the emphasis could have been more on other characters’ reactions to the slaughtered bodies left in the Parasytes’ wake, and less on the slaughter-in-progress. But that would have lessened the impact of the main character’s situation. Shinichi Izumi is, quite literally, stuck in symbiosis with a shapeshifting alien who missed being a man-eater by inches. And they have to deal with the other Parasytes who didn’t miss, and see a human in the know as a threat to their survival. Staying alive in this setting has a price, and it’s paid in blood.
Rule of thumb, if you’re not doing a story with a horror aspect, you’re probably not using as much blood. Most mystery stories have to start with a dead body, but poison, electrocution, and drowning are just as corpsifying. You may prefer a non-bloody death in a cozy mystery, for example, because it might be passed off as “just an accident”, leaving an excuse for an amateur detective to poke around when the police don’t.
Fantasy and sci-fi… it probably depends on where you want your story to hit on the Sliding Scale of Idealism vs. Cynicism. The more idealistic a story is, the more likely the characters will be able to resolve things without buckets of blood everywhere; and if someone is injured, healing magic/tech can patch them up. The more gritty and grimdark, the more likely you are to have people scarred, losing eyes and limbs, and bleeding out.
(Note: I’d be interested in seeing a fantasy that – along with healing magic – also had some form of our modern “stopgap measures to get someone to the medics alive”. Like tourniquets, gluing wounds together, and expanding sealant to stop blood loss.)
What do you want your reader to feel when people bleed? Sorrow? Terror? Nausea?
Think it over. Hopefully before the plot ninjas swoop in and slaughter a character in a way you can’t write yourself out of!
(Side note: Stitches are aggravating and awkward, and none of the action books I’ve read where a character got stitched up cover just how annoying it is. Realism fail!)