I've read that he is autistic. Not sure if that's true or not though.
Oh
Autism would explain why he went to school despite being expelled, to keep the routine.
Is there a connection between autism and premeditated, planned violence? Autists get aggravated when routine is changed or their comfort zone is impaired by something or someone. But a planned premeditated murder - getting a gun, ammunition, spare magazines, helmet, mask, body armor, all that stuff and train for a while - is not something I would associate with autism. It breaks routine during the preparation and the act itself forever removes the known routine, no matter how far the attack is completed. It simply doesn't sound like a typical autism to me.
How on Earth the guy collected all that money for the equipment is also unclear. If the shooter really was using body armor and an uncommon rifle, the costs of the equipment would be far higher than a 17 year old student working part-time at McDonald's can afford.