IMO it would be a justified shoot. He still had the opportunity to clear the gun, and shoot the clerk, or others.

Personally I do not carry to be a hero, I carry to protect my family, and myself. If the robber appears to be a threat to us then I would take action. If the robber shoots the clerk, then it is a good bet he is a threat to us. If he just takes the money, and leaves it is not my concern.