Until he posted the You Tube video, what had he said or done to justify another visit from authorities, arrest or forcible admission to mental health?
Assuming he was serious at that time, “How to you stop someone who has murder in his heart.”
How would we know all that he’s done or said. There seems to be no shortage of threats, violence and erratic behavior. The neighbors were sure eager to talk and glad that he moved away. If he is threatening others and brought ammo and knives to school, he was subject to arrest.
The system had already failed him repeatedly and systematically for over a decade. He likely should have been institutionalized at a much earlier age. Whatever he did at school should have been reported to the police and juvenile hall.
He should been arrested for some of his earlier assaults against his brother, mother and for cruelty to animals. His neighbors could have also prosecuted him.