Honestly I think it's a function of how he wants to present. I am reading that this boy attacked people with a hockey stick, and armed himself with knives. He seems to have been preoccupied with revenge and violence. The question is really why he was at home. The story of his father dissuading him from going out with a knife is very concerning.
I was a victim of an attack like this, threatened with a knife in a child's playground. Lots of children and families. The man was highly disturbed. I saw him run towards the playground with this huge knife, after he had to been persuaded to leave. I had to tell at everyone to get out. I met the offenders father on the way out of the playground.
But he was able to rob and terrify people without being stopped. The police had to taser him to stop him killing someone. He is in prison now.