And when they start showing disturbing behavior, it’s ignored. Clearly signs were ignored prior to this, or at minimum,
unable to be fully addressed, giving him a slap on the wrist because of his age just enables him to continue down a destructive path.
First off, none of that sounds like "turn the other cheek", which you claimed is what creates serial killers.
Second, what are you basing all of these claims on? The article I linked stated that there are two primary pathways to child psychopaths. One is the result of abuse - the majority are kids who are severely beaten and emotionally neglected as children. The other path is apparently genetic and occurs without abuse, but punishment makes literally no difference to them. It explicitly says in the article that their behavior isn't impacted by punishment at all. Therefore either way, withholding punishment from a kid is never going to create a psychopath.
And you say, "signs were ignored or unable to be fully addressed", but yeah, that happens all the time. The first case study in the article is about a little girl who showed clear symptoms even worse than this kid for 4 years before she was diagnosed. Her parents appear to be a caring, conscientious middle-class family who was completely on top of everything, who saw the warning signs, who took her to psychologist after psychologist, and it still took nearly FOUR YEARS for her to be diagnosed without any of the interventions before then helping at all. And that's a worst-case behavior girl in a best-case family situation. The average poor or working-class single mother doesn't understand child psychology or know how to navigate the systems to get help in the same way, or maybe she did try to get help but he was misdiagnosed. There's no justification for claiming that she just tried to "turn the other cheek."
The fact that she had a gun in the home makes the idea that she was some sort of "turn the other cheek" practitioner pretty unlikely. And THAT mistake, having the gun in a home with a child who clearly has some issues going on, is by far the most obvious mistake in the entire sad episode.