Artifact - now that's an academic for you.
I will opine that training is important in the high intensity events. In two rampages, we see the civilian good samaritan having troubles.
Tacoma Mall - the civilian can't take a shot and is paralyzed. For some reason, he froze at the action moment (I could give an opinion as to why - probably couldn't overcome the tension/fear block on acting against another).
Tyler, TX courthouse - Good Samaritan hits the BG who goes down. Instead of hunkering down and/or approaching cautiously, he goes up to the armored BG who kills him. Bad tactics.
Either might have benefited from more intensive simulation training than they seemed to have had.
In most of the DGUs, showing the gun makes the BG go away. In most of the shots fired ones, the BG ceases action. These are probably the economically motivated crimes where the crime going awry doesn't justify the risk of harm to the BG. In the intense incident with emotionally or more motivated actors, then training is probably needed.