Thanks everyone! One of the goals here was to determine self defense. Is it just yourself or the defense of other innocents. From most of the feedback, it's yourself and let the fate determine the clerk's life.
Well none of that is based on any sort of real life case law so if that's the conclusion you draw go right ahead. I have a hard time believing a jury would convict you for smoking some dude who had a gun in a cashiers face, make that doubly so if you have no ability to reach the exit, which is likely the case since the cash register is usually by the door.
As I said, every situation is unique. I carry a gun to defend myself, not play cop. I'm also not going to watch some innocent person get murdered if I can do something about it (gun or no gun). If you can live with that the rest of your life, that's on you.
Mike