If he started shooting the employees I would say engage the threat. If he doesn't display a weapon and you shoot him, you may have just shot an unarmed man (the old finger in coat pocket gag). Besides, if he wasn't going to shoot and you pull out your gun and yell FREEEEZE, he might decide to start shooting.
If he is dumb enough to rob at gunpoint sans gun, chances are he is not clever enough to make a clean getaway. Give a good description to the cops.