The heshe had something against the adults targeted for 20+ years. The 3 kids were collateral. If the goal was numbers and "fame" those 14 minutes would have been way worse.

Why was it unable to open lower level doors, though it shot through the glass entry, yet was easily able to access the targeted adults on the second floor?

Don't administration follow lockdown procedures, or do they just walk out in the hallway unarmed during a lockdown? One is seen in the bodycam video in the hallway, already shot.