After the robber jumped the counter he went for the merchandise not the clerk.The clerk went to the robber.
In that moment, what would YOU have done? You don't know, because you have never been in that moment. Unless you have and have not told us. Adrenaline, fear of one's life, etc etc. If that was Florida in the 2000s, cut and dry case. Forcible felony stand your ground shit. You were allowed to use deadly force to protect property. I do not know what it is now, and I am sure this case does not have such protections.