BG flashes weapon? What are your choices?

I'm sure this topic has been covered before but if so I do not remember reading it and a search of the forums did not pull up an answer.

If you are at work, out shopping or approached on the street and a BG lifts his shirt and flashes a weapon what are you allowed and not allowed to do?

Does the weapon actually have to be out and aimed at a person before you are allowed to draw your weapon and defend yourself?

On another thread it discussed a Dollar General store being robbed and the employees and customers were herded into the back room.

If you were there and robber had lifted his shirt flashing a weapon but did not actually draw it what would you do?

Would you take the chance that he/she would just collect the money and leave and not further escalate the situation

or

would you not take the chance and fire on the assailant knowing at any moment he could change his mind about leaving witnesses and execute everyone.