The guard shoots a proto-adult thug that possibly is in the starting process of mowing down people 'ala Charlottesville because the Lab probably tested his pee and he failed. Guard didn't check the perp's ID (too young to have a Driver's Lic.), and I guess the guard didn't see the "Don't Shoot me I'm only 15, but bigger than you" neon sign attached to the guy's forehead. I guess the guard was so incompetent he didn't personally know this kid or where he lived, so that he would know he was going home and should have memorized all the names, home addresses, and ages of all students.
Obviously it's another example of law enforcement gone rogue