"You need to get out of here before I blow your head off."
Branca, a lawyer and expert on the Law of Self Defense, provides his perspective on the unfolding situation with a step-by-step analysis. One of the more cogent points he makes is:
It's worth noting that this is how the vast majority of defensive uses of firearms end – without a shot being fired, the defensive display of the firearm being sufficient to defuse the situation without injury to either party.
Branca goes on to write about the elements necessary to determine a proper armed defense response, points that one should grasp well prior to any such situation because it is typically spontaneous. This leads to the usual 'what if' discussion revolving around the question of "Would the clerk have been justified in firing on the robber?" The robber drawing his weapon and stating that the clerk needs to give him "all his money" would constitute a reasonable conclusion in my mind that the robber meant imminent harm, thereby granting permission to the clerk to blow him away by any means at his disposal.
The comments section contains some good discussion as well – avail yourself of it (other than a needlessly prolonged 'who has the bigger gun' argument – and I mean 'gun' euphemistically).
For my own input, it would appear that the robber would be 'under the influence', which to my mind would affect the reaction, and note that the clerk is carrying openly, with his handgun (apparently a Glock or variation) in a molded holster – Blackhawk or some such. One would think that any self-respecting robber would want to notice that before starting a confrontation. Other than gun stores hereabouts, I haven't seen store clerks carrying in a manner to provide optimum protection to themselves and the public, but then I live in a indigo-blue state that loves to intimidate the populace to make up for the fact that there are rather sensible laws left on the books from a time when the political climate involved more outside-the-beltway reality.
Remember, the police show up only after a crime is committed.
H/T to the Texas Scribbler.