Use of any weapon in self defense, including one's fists and feet, is only justifiable in most U.S. jurisdictions if the "reasonable man" test applies.  If it comes down to defending the use of force in a court of law (or to your own conscience) then it will need to be accepted by most impartial people that given the circumstances, any reasonable person would have done the same thing.  

All armed citizens, including armed law enforcement officers who conceivably may one day apply deadly force to save themselves or others, must take this credo to heart.