Supposedly the guy with the gun was antifa.
I don’t know why you would bring an AR15 to a peaceful protest. 
Trying to piece together what I've read about the SLC shooting that was less than clear:
* The event organizer had paid armed "security" people (licensed? unlicensed?

) at the event;
* Some Prog rando dude independently decided MAGA people would be there and would be violent,
* Rando dude took a rifle to the event area and went to a place of concealment near the periphery of the event;
* Rando dude moved from his place of cover, was seen by a "security" person(s);
* "Security" person(s) fired shots more or less at rando dude;
* Rando dude was wounded, and a person who may or may not have been an event participant was hit and killed.
If I'm incorrect, do set me straight, because I'm kind of fuzzy on what went down. It kind of sounds like a little bit of Keystone Kops with real bullets.