A person can use deadly physical force when they reasonably believe it necessary to defend themselves or others from the use or imminent use of deadly physical force on themselves or others.  Deadly physical force means physical force which can cause death or serious physical injury.  To use deadly physical force, a person must reasonably believe that someone was using or about to use deadly physical force against them or others, AND that the use of deadly physical force was necessary to defend them or others, AND a reasonable person in the defendants situation, knowing what the defendant knew, would have had the same belief that deadly physical force was necessary.  This defense is not available if the defendant was the initial aggressor.