Initially I was going to reply that ...
You try to keep your "Situational Awareness" active as much as possible and learn to look for warning signs so you can try not to end up in a position where someone can just "stick a gun in your chest." Criminals are very good at gaining access to do something like that, though, so avoidance is not possible in every case.
If someone has a gun pulled on you, you are largely STUCK. "Drawing to a drawn gun" is a losing hand no matter how you play it. Now if your assailant is extremely close and you are very good at hand-to-hand extreme close quarters fighting and you really think you MUST fight or die, then you may be able to disarm him, but a very skilled individual would call that a total roll of the dice, at best.
Your gun is not the only "tool in your toolbox." You've lived this long on your social skills -- i.e. your ability to communicate, understand people's intentions, and express your own. If someone has totally dominated the situation by covering you with a weapon, that's a good time to give them what they say they want and look for the safest path out. Stuff can be replaced, and MOST criminals aren't looking for a murder rap, just some free stuff. This is NOT SAFE. Some criminals will shoot someone for the most tenuous reasons. But it's a better bet than trying to "slap leather" and being shot, or playing cutsie games and getting shot, or playing tough and getting shot ... (see the pattern?).
In this case, the guy did EXACTLY the right thing. He is alive. His stuff can easily be replaced. He didn't fire a shot that killed a bystander. He didn't even fire a shot that ended another man's life (the robber) with all the attendant unpleasantness (police, arrest, lawyers' costs, court, depression, mental anguish, etc.) that comes with that.
He's fine and he's free and life absolutely goes on.
It *might* not have gone that way for him, but by the time he realized he was in trouble, there was very little else he could have done. The criminal could have killed him outright, and there's some infinitesimal chance that he could have drawn a gun and defended himself before that happened. But it didn't. This time, everything went as well as it could have gone.
I'm not talking about when he was approached, nothing you can do unless your weapon was in you hand. However afterwards, when the suspect left and went to the car, got in and pulled away, there was ample time to "do something". I'm just not sure what I would have done. Runaway to safety seems like a good option.
BUT -- Ok, this changes your question a lot.
You do not have the right to use lethal force on someone for HAVING committed a crime. You do not have the right to use lethal force on someone who is LEAVING the scene of a crime. You do not have the right to use lethal force on someone for vengeance, pride, honor, or to try and recover stolen goods (
with one exception in Texas, at night, etc).
You have the right to use potentially lethal force to STOP SOMEONE FROM KILLING YOU. (Or from gravely injuring you, or a couple of other extremely serious felonies.) That's IT. If someone is leaving in the car they just stole from you, they AREN'T still trying to kill you. If someone is fleeing from your home with your TV, they AREN'T still trying to kill you.
Doing something? What COULD you do? Fire a shot? Your justification under the law for using deadly force on another human being is GONE, if he's leaving. You are now committing assault with a deadly weapon and/or attempted murder. It really is that black and white.
What could you do? Get to safety, call the police, call your insurance agent, call for a cab. Sit down on the curb and throw up for a while (which is probably what most folks will do, once the adrenaline starts leaving). Thank (whomever you wish) that you're still alive and unharmed.
