So if I'm reading this right, there were armed peacekeeping volunteers organized with the event (not necessarily by the protest organizers but they were part of the protest). One of them saw a guy grab an AR-15 from an alley and confronted him. The rifle-weilder raised his rifle, the peacekeeper fired several shots, killing a random person and wounding the rifleman. The rifleman is being charged with the murder because Utah law probably counts anybody dying as the result of a crime to be murder for the person committing (a few states do that). They're currently investigating the peacekeeper.

I'm curious to know more about this peacekeeper business. Were they openly armed? Were they all armed? How big was this group? How were they organized? Presumably the rifleman was trying to start shooting or at least threatening to shoot. If the police weren't sure of that much, they wouldn't have charged him with murder. If the peacekeeper isn't even being arrested, my guess is the police feel very confident about how this went down.