The purpose of a defensive firearm is protect a person from assault by bigger stronger thugs, who do not need a weapon to kill you.
To a mugger, the ideal victim is weak, elderly, female, or otherwise easily overpowered without need of weapons, because assault with a weapon results in more serious charges than using your fists and feet. And as SimpleMan points out, a strong man in his prime can kill a weaker victim with just fists and feet.