This is why "dark" YA books exist.
A local story in the Philly area: A man kills his own son, but it doesn't appear to be a (stereo)typical murder or family conflict.
(WARNING: do not read the comments on that article. The readers of Philly.com are, shall we say, the dumbest people on Earth.)
tl;dr version: A police officer who, by all accounts, is an easygoing, laid-back guy, shot and killed his teenage son who was attacking him with a knife. The son was also, according to those who knew him, a sweet kid. This is a solid, middle-class, white, suburban family, the sort of family the "think of the children!" crowd would call "normal."
The twist: the boy had recently been involuntarily committed for attempting some sort of self-harm. He was released just a couple of days before these events.
There are a lot of details missing, of course. On the surface it seems incomprehensible. But it highlights a common situation: however nice a family appears to outsiders, there are likely some issues that they are dealing with.
As a writer, I start to frame how I might write a novel around this. I can ascribe any number of motives to both the father and son. The book could talk about the terrible toll of mental illness. Or it could be an after-the-horror story about how a man deals with having to kill his own son. Or perhaps there could be some hidden issue that is being covered with a fiction of "mental illness," and one could write a story where a perfect family is trying to cover their son's drug use, or a family conflict over homosexuality.
Whatever the ultimate situation, when things like this happen in real life, even adults are at sea to make sense of it. Imagine how the boy's classmates feel. And perhaps if they had books to read, fictions to relate to and give them some anchor, they would have the beginnings of the tools they need to process this and grieve and cope.
Bad stuff happens, and it rarely makes any damn sense at all. Fiction is how we learn about humanity, and one of the important functions of fiction is to teach people that bad stuff happens, but this is both (a) normal, and (b) survivable.
One of the advantages to getting older is experience. I've seen or read about enough crap that I like to think I have some decent collection of coping mechanisms when crap hits me. (I've been lucky to not personally suffer a lot of crap.) Teens are entering a world where crap hits (if they haven't been living in such a one already), and if we send them out without even theoretical knowledge, they'll have a very hard time coping.