I've been thinking about this a lot today. I'm dealing with how much violence to include in my stories myself.
I recently read a YA book that includes a brief, non-graphic but highly evocative description of suicide. It is integral and necessary to the story. I have no objection to the writing-in fact I admire it. It has also been on my mind all day. It was on some level traumatic to read, even as an adult.
I don't remember the exact age I was when I read A Swiftly Tilting Planet by Madeline L'Engle. I loved that book like crazy at the time, and I still love it now. It contains one brief paragraph about a man's experiences in the civil war. There's a single graphic sentence: "Matt, I saw a man with his face blown off and no mouth to scream with, and yet he screamed and could not die." When I read that sentence, it haunted me. I read A Swiftly Tilting Planet over and over, but I always made sure to skip that page--which was funny, because I knew the sentence nearly word-perfect from having read it just once. (In fact, just now I typed it in blind. Then I looked it up and had to add the name "Matt" and change "who had" to "with.")
Am I sorry I read that sentence? No. It's not possible to avoid all the trauma in the world. Did it shape my adult view of war? My political views? Who knows.
My point is that there's a tendency to think of this issue in terms of wanting to know what the rules are. Or maybe that's just me. But another way to look at it is that stories have their own integrity. Reading about realistic violence is traumatic. A good story balances that trauma with the purpose behind the trauma. A "fun" story won't hit the reader with a traumatic experience. A serious one will, but only to the extent that it needs to.
"Fun" stories have a huge amount of leeway to engage in swashbuckling swordfighting horse-racing galaxy-spanning dust-ups where somehow nobody ever gets seriously hurt. But that gets complicated too, because the same story can shift from "fun" at one moment to deadly serious during another. Plus the desire to spare the reader leads to an odd message: that nobody important ever gets hurt. (Cue jokes about Star Trek and redshirts.)
A Swiftly Tilting Planet is in part a story about a telepathic boy who travels through time on a winged unicorn to save the world - and my younger self for sure found that to be a sweet, sweet fantasy. But it's also a serious story about what sways people to good or evil actions. The keystone threat is that the world would perish in a nuclear holocaust, which is something that quite literally terrified me during that period of history. A certain amount of brutality is fitting to the theme. But if I had to read page after page of civil war gore, I would have shut the book.
Anyway, this isn't really the sort of post that has conclusions. Just stuff that's been on my mind...