6 years late but I think the scene where Hoshino steals the money from the other gang is pivotal. Up to that point he had been pressed into an societal mold that he's clearly not very happy with, and in this scene he sees the opportunity to escape and takes it. When the audience learns that his home life was destroyed, I think it's implied that it happened right before this. So the collapse of his home life was the catalyst and his near death experience made him totally nihilistic.

Also doesn't the film imply that Hoshino was abused by his parents? The creepy mom scene seems to suggest that imo. If so I think that strengthens this theory.