If you were a sincere martial artist, you would do everything you could to avoid a fight. As the defender, your main objective is survive! The U.S. Constitution gives you the right to defend yourself. Obviously, if the other person called you names or spit on you. Deadly force would not be authorized! However, if the confrontation escalated to punches and kicks. If you were losing badly, and you felt your life was in imminent danger. You would have to do whatever is necessary to stay alive.
If, you killed the other guy, first call an attorney, and do not answer any quesitons. That is also your right. If you are arrested and brought to trial. You would have to justify your actions to a jury and make it believable! A step by step account of your actions will be necessary.
Regardless if the attacker was a martial artist or not. Taking a life is a terrible decision. The outcome could be fatal. For you and the other guy.
The Code of Bushido. The Samurai Code says; run before fighting, fight be injuring, injure before crippling, cripple before killing, kill or be killed.
Retreat is the best option! Call the police as soon as possible and report the incident. You alone have to make the final decision.
Rather tried by 12, then carried by 6.