The guy who got his bicep blown off, is a felon-in-possession; and was photographed right in front of the 17 year old, just before the shooting, with a Glock in his hand; and in a closeup of what's left of his arm maybe 10 seconds later, still holding the Glock.
He's wearing an "Paramedic" cap too, which is odd for someone armed with a handgun. (And in the "after" photo, he has a *different* cap stuffed into his pants leg. Why?)