I keep seeing videos of thugs shooting without warning... and I keep hearing arguments like this one. Don't forget the time factor with its implicit threat.
If the BG is in front of you and reaching under his shirt, while trying to heist your store, there is a reasonable possiblility he can and would wound you mortally in about a second, with knife or gun.
Just because the bozo has no weapon out yet, does NOT mean you should wait till he points it in your face before you decide you should defend yourself. By then you might conceivably have about a third of a second before your fatal event.
I hate to hear of anyone dying when they had no weapon, but only acted that way.
There must be some kind of guideline here some are missing... Perhaps one should not look, sound and act like a thug who would take an innocent life in an uncaring blink of an eye.
Of course that's just my opinion and I could be wrong.