Well, with a little background, the picture is starting to become a little more clear. Not a ton, but a little.
First, the shooter went to an alternative school for troubled kids. He was there waiting for the bus to take him to that school. It sounds like the kids he targeted also went to the alternative school. So it seems like a specific beef with those kids, maybe bullying (that'd be my guess).
Secondly, it sounds like his home life is a wreck. The kid lived with his grandparents. His dad has been in an out of jail for a whole host of violent offenses, including assaulting the kid's mom, assaulting a police officer and holding another woman under water while bashing her head into a wall. The mom had also been arrested in the past for domestic violence against the dad.
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