We've seen vigilante justice celebrated before in America - see Trayvon Martin getting murdered by George Zimmerman. Half of the country thought it was ok to kill a little boy in a hoody because a grown man was scared. Zimmerman walks around as a free man today. Mainly because SOME of the public was ok with the murder.
This shooter had support from most people BEFORE anyone knew a thing.
I've seen people online theorizing that Luigi had major chronic pain from a botched back surgery and that it likely drove him crazy. The public is already building his defense.