Based on the original story
and question:
What do you do if you're packing in this case.
Be a good witness. Get back in the store where there is cover. Defend only if attacked or if I can see that the shooter is aiming at innocents.
Based on what the guy heard (shots) and saw (an older guy, aiming in a specific direction), I wouldn't do anything to engage with him. I don't know why he fired. I'm not even sure it was him doing the firing. He might have been shot at by the motorcyclist and drew his own gun but didn't have an opportunity to shoot before the biker left.
Coming upon a situation, or having our attention drawn to an already ongoing situation, leaves us woefully ignorant of very important details. He might be a cop, he might not. He might have been the one shooting, he might have been shot at. He might have been defending himself from a carjacker. Way too many unknowns to take any aggressive actions.
But for those advocating aggressively taking him out, you don't absolutely
KNOW what you've walked into, so don't make it much, much worse.
I'll throw out just one possible situation: He just defended himself from an armed robber, his adrenaline is sky high. You come up behind him ordering him to drop the gun. He's got tunnel vision, his ears are ringing from several gun shots fired without hearing protection, and his focus is on the motorcycle. He vaguely hears your commands, and turns slightly toward you with a "huh?". The gun is still in his hand, but isn't necessarily pointing at you. But he may be thinking you're another armed thug. From his perspective, it's even probable. So if he does try to defend himself, legitimately, and you shoot him, who's the winner? You will have interjected yourself into a situation about which you did not have all the facts. Yes, legally, you may even be cleared of charges. But, and this is important, you should never have been in a position to have to face charges. Your ignorant (and by that I mean "not having ALL of the facts") charging into the situation would have left a good guy dead and you bankrupt from the civil suit sure to follow.
Always KNOW that you are absolutely right in pulling the trigger, and that the pulling of that trigger is the only option left.