In our CCW class, they taught us if the threat is real enough to pull the weapon, you use it. Displaying a gun with the intent to cause fear is brandishing a deadly weapon. Ironically, you can be criminally charged for doing what you described. My thought is, if your argument is that you're in fear for your life (multiple assailants running towards you) then deadly force is justified - and withholding that force raises doubt as to whether you really felt death or grave bodily harm was imminent.
I don't want the death of another person on my conscience, but two guys looking like that, charging at me, they get no warning.