Because it takes time for bullets to have an effect, guns can in fact be emptied before the the person being shot even starts to fall down. A revolver can be emptied in less than a second, though most people are not that fast.
I know all that but...If the perception of a threat is wrong and the shootee is dead with multiple gunshot wounds the shooter is in deep chocolate.
Even when the threat is real. Why empty your weapon? It seems like a vulnerable position to be in if the perp has a friend lurking in the shadows.