I suspect that these acts of violence are spawned by some form of abuse.
Abuse can take many forms, and individuals react differently to abuse.
Abused people sometimes resort to aggressive anti-social behavior. One form of abuse can be parental neglect, another form of abuse can be parental indulgence. All abuse has negative consequences whether it causes an abused person to withdraw from society, or whether it causes him of her to strike out against society. In this case the perpetrator apparently wasn't a bully, but rather he was the object of disrespect. One of the students at the school said he had been really distressed and that he didn't have very many friends.