The "fair fight" concept doesn't apply when it involves a law officer. If a policeman gets attacked, then I think he can punch whomever (man, woman, child, animal, etc). The same thing applies when a regular person gets attacked by someone intent on killing them especially if the attacker is armed.
The fairness principle I was referring to should mainly be used in other areas. ex: When two drunk guys are fighting each other and neither is in mortal danger. In those type of situations its wrong for guy to fight or punch a woman, but not as wrong for one guy to fight another guy.