My best friend is not an officer. He is, however, a master class IDPA and USPSA competitor and firearms instructor. He and I hit most of the IDPA and USPSA matches in AZ together, and train together and now that I am retired, we often teach together. I am sure of his competence to act in defense of himself and others.
That being said...he has no experience as an officer, and we have discussed this subject before.
My view is that (pre-supposing I was still active and not retired LE) in an off duty incident such as the OP has described outside my jurisdiction...I would be a good witness. I would only act if I was convinced to my own satisfaction that violence was about to ensue.
The reasons for this are many, but to hit the highlights:
Let's assume the clerk has hit a panic button or silent alarm and officers are en route from the jurisdiction we are in. Smoking a subject as they roll up, not knowing who WE are, could be a recipe to catch (un)friendly fire from the other "good guys". All the responding officers know is that two big guys with guns just shot another human being in the location were an armed robbery had been reported.
The fluid nature of an armed robbery (such as other customers walking in while the robbery is in progress could put innocents in the line of defensive fire, not to mention fire from the subject as well) is such that intervention on the part of an armed citizen, which we would both be in this instance, could endanger more people than it would potentially save. Again, this is predicated on the fact that I am not convinced to my own satisfaction that violence is about to ensue.
I hate to invoke "statistics"...but they do point to the fact that as long as the subject meets no resistance in a convenience store robbery, they will get the money and run. No, this is not 100% true 100% of the time, but in the vast majority of cases of which I am personally aware.
It also depends upon how the subject is armed. If they have a drawn gun...then the situation is more complicated.
If they have a knife, and there is a physical barrier (the counter) between them and the clerk, then you have more freedom to act to stop the robbery, but again this is dependent upon your level of skill. In this situation, then I believe I would draw and issue a warning to stop and drop the weapon. They are still committing an armed robbery and I don't want another customer with no barrier between them and the subject to get stabbed on the way out, or trying to "wrestle the knife away" from the subject. In this case, I believe that customers are in more immediate danger than the clerk within arm's length, or the Tueller Distance.
As an officer, and a trained observer...If I see any gang or prison ink on the subject, then I have access to data bases in gang units that will help in the identification of the subject. There is even a decent chance that I might recognize the subject if they are a regular customer of the justice system. If not, I will damned sure remember them and may be able to hook them up on a later date without endangering bystanders.
No one wants to get killed for the forty bucks or less in a store's register. No one with any sense wants to kill someone over that forty bucks or less.
The time for intervention is that if you honestly believe someone is in immediate and otherwise unavoidable danger of death or grave bodily harm.
I am not saying "never get involved"...but what I am saying is be pretty damned sure that your intervention is going to help more than hurt.