While answering the hypothetical situations is always a bit of a guessing game, here goes. Based on the scenario you presented the BG doesn't know you are there, if you are discovered it may well escalate the situation. Your best recourse would appear to be to retreat to a position of cover, draw and cover the BG, assess the situation, is there anyone else there, what is the emotional state of the BG, does it appear they are about to shoot or is the gun more of a prop (I know this could go either way but something to consider) etc. Forget you have the cell phone, the time for talking on the phone is past, you do not want to divide your attention between the threat area and a dispatcher plus it increases your chances of discovery by the BG. Now if you are able to retreat from the building to a safe location by all means do so and call 911 but in the store is not the place. After briefly evaluating the situation you must make the decision to wait or to engage, personally if I am in the situation you set forth and I decide to engage, I will probably not challenge the BG I will probably just shoot (I know the law varies on this from area to area and you must follow the laws that apply to you as a CCW, my agency policy which governs my response says I don't have to if doing so endangers myself or another which in this case I think it would). If you do engage maintain a position of cover until the Police arrive, more than one person has been killed approaching a "dead" BG. Make sure that your weapon is put away as soon as you see the Police arrive on scene and carefullly follow their instructions it is literally life or death important. If you decide that the situation does not warrant shooting then remain in your position of cover until the BG has left and concentrate on being the best witness you can if you decide to get involved when the Police arrive, once again make sure your firearm is out of sight when they arrive. One thing is for sure anyone who carries a gun needs to think about these things before they are in the situation, playing "what if" helps you to stay alert to your surroundings and to be thinking in advance of a plan for a given situation.