... you said yourself, in many jurisdictions, it's a threat to oneself OR OTHERS.
If the guy turns his back on me, but is still threatening the clerk, then it's quite possible that a shooting would be legally justified.
Whether it would be tactically sound is another matter entirely.
And one had better be extremely sure one understands the situation before defending a stranger; sometimes, the apparent BG is a plainclothes cop making an arrest, for instance.
But plainclothes cops don't typically hold up convenience stores.