This video shows why you keep engaging the threat until the threat stops, or you run out of bullets. Notice how the perp has been hit, at least twice but still gets one of the guns and attempts to shoot one of the clerks. This ended well for the clerks, but it could have gone sideways very quickly. He should of been dropped when he turned the corner after they armed themselves.
http://www.breitbart.com/big-government/2018/02/24/watch-mother-daughter-in-gun-fight-with-shotgun-wielding-robbery-suspect/