This is one of the perennial scenarios discussed here, and the discussions usually start with the question of whether deadly force would be justified (ans: yes) and end with this, restated in Post #5 by 9mmepiphany:
The usually recommended course is to be a good witness unless there is an overt act that would lead a common person to believe someone was going to be mortally injured...and even then, you'd have to have a clear shot.
Post #5 continues with an analysis of possible outcomes:
1. You make a good shot and he stops being a threat - good
2. You make a pretty good shot and he runs away - good
3. You make a good shot, his gun goes off and he kills someone - bad
4. You make a pretty good shot and he starts shooting everyone - bad
5. You make a bad shot, miss and he starts shooting everyone - bad
6. You make a bad shot and hit an innocent bystander - bad, plus the situation goes downhill after that
Worst case - You make a good shot and someone else who is also CCW shoots you thinking you are another bad guy.
There is, of course, one more: the robber's "tail gunner" sees you draw and shoots you.
Those who would advocate shooting usually envision outcome #1 as the likely result of one's decision to shoot, but the possibility of the robber moving at the last moment, the fact that the person who intervenes will have to draw, aim and fire very, very quickly in a high stress situation, and the fact that handgun bullets are unlikely to have the effectiveness will that many may expect, makes that outcome less than likely--or, as Flfiremedic says in Post #11,
Saying you would neutralize the threat if a high chance of success is found, is pretty bold.
For a dose of reality, read what GEM has to say in Post #12:
While it is easy to proclaim you will draw your gun and take the guy out, I wonder if you have tried such in well designed FOF exercises.
You may talk the moral talk but you should try it.
Before you say that you will draw your gun and take the guy out, you have moral responsibilty to have a reasonable amount of training to act well. It's not like shooting at a stationary B-27 with the guys and chortling.
I suggest that after anyone has tried a well designed FoF exercise and considered the effects of real world stress, he or she will think twice about the idea of intervening. Make that three times, if one does not know for sure who and where a possible accomplice may be.
In Post #8, Flfiremedic poses several important questions to consider:
Unless there is an immediate and articulable threat to your life and or the life of another, why would you escalate the situation? Yes he may have a gun out, but has he fired? Singled out/selected targets?
How many bystanders? Have you identified his partner? Has he identified you as a threat? Can you reasonably draw and fire even if it is appropriate-before being fired upon? Have you cover/concealment? What kind of back stop do you have? Range? Nearby innocents?
Post #8 concludes with an excellent recommendation:
You should be in a hyper-vigilant mode, so concentrate on remembering everything...race, skin tone, clothes, tats, hair style, hair color, direction of travel, means of travel EVERYTHING you can remember...in other words be a GOOD WITNESS!
My opinion? Follow that advice
unless (1) the robber starts shooting or (2) the robber orders everyone into a back room, either of which takes back to Post #5's reference to an overt overt act that would lead a common person to believe someone was going to be mortally injured.