[House Atreides]

“...the gunman walked into the store waving a gun in the air and forced a cashier and customer at gunpoint towards a break room area....”
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Break rooms and refrigerated rooms are where perps, with murder(s) planned, take victims to be killed. I trust the armed customer gave the perp a double-tap to the chest followed by one to the head—the situation called for that in my opinion.

[ThunderSleeps]

...the customer pulled out a concealed weapon and shot the suspect in the chest once. The gunman was pronounced dead at the scene.

That was some mighty fine shooting. One shot, no question, DRT.

My SOP when carrying my 9mm is (barring special circumstances) if I shoot, I shoot #1 center mass and immediately follow up with #2 to the head. Then I'll evaluate the target. I figure the center mass is the high percentage shot, while the head shot has the biggest chance of stopping the perp in their tracks. If I'm carrying my .45 (a full size S&W stainless 1911) I'll go with just the center-mass shot then evaluate.