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)
So what? SO WHAT?
If he wants to make a reasonable person believe that he has a gun, then a reasonable armed person will proceed on that premise rather than trying to read minds.
Besides, if he wasn't going to shoot and you pull out your gun and yell FREEEEZE, he might decide to start shooting.
Besides, if he was
going to shoot you and you didn't
pull out your gun, you'd be about 21 feet behind schedule.
This is not the appropriate stance to take with a violent criminal: