What are you supposed to do if someone lifts their shirt to show you their gun in a threatening manner while you're carrying?

A neighbor told me he was at a convenience store and someone started yapping item cuz he wasn't moving fast enough in line when he turned around the guy lifted his shirt to show a handgun in his waistband. As a concealed carrier what would you do? The neighbor said he just backed away from the guy shaking his head and left without buying anything. His encounter got me thinking what I would do in his situation, I think you handled it well and got off safely what are you supposed to wait for someone who obviously has a gun and is now threatening you to draw and shoot you? He said the guy kept talking as he walked out of the store for what if the guy would have chased him into the parking lot maybe not aggressively but just follow them? via /r/CCW https://ift.tt/305YNwp