I have learned that one who is licensed to carry must have as much, or more resolve in their head as they do in being proficient with their firearm. I wouldn't bear if I were just mad, or expecially in most 'third party' situations. If I were in the back of a convience store and two guys entered with the intentions or rob the place and pulled a weapon, and I was carrying, what should I do? I think that 'pre-thinking' is on order here but I wouldn't have much choice if one of them came towards me with a weapon and obviously were going to us it. So, in short, it depends.
I've been told by professionals that it is better to be a good witness than to try and intervene. If possible, retreat to the back of the store, go out the back, in the bathroom, the cooler, the office, or anywhere that's out of sight of the robber(s). If they have seen you, and sometimes they scope out the place before bearing, and notice that you have gone, they will either look for you or leave quicker. If you are, say, in the bathroom calling 911, you would have the advantage if they were to come back to look for you. Do what you have to do. If you are at the front counter, back off and if confronted, say something like 'hey, I don't have anything in this', stay very still, and hold your hands up, give them your wallet if they ask for it, etc. Remember that their fear level is above their ears so don't make any sudden moves. If they start shooting people in the store, which is unlikely since they only want the money, then you must try to protect yourself. Even a cop would call for backup outside so it would most likely be a mistake to 'play cop' inside by yourself.
In a 'third party situation, like an a parking lot, you come out of a store and see a man shoot another, how do you know that he isn't an off-duty officer, or another CCW person like you that was just confronted by deadly force? Now, if they start directing that smoke wagon at you, that's another story if you have the space and time to draw your CCW. And remember that you are responsible for all damages or deaths that would occur near or behind this person.
You better think twice, and be mentally trained to think twice very quickly. Good question