Legally, he doesn't have to pose a threat to YOU. It is legal to use deadly force to prevent someone from using deadly force against another (illegally) if I'm not mistaken.
Legality has nothing to do with whether its right or not.
Would anyone here change their answer if the person was being stabbed with a large knife rather than being strangled by a cord? Serious question.
Would I shoot him if he was "being stabbed" yes.. about to be stabbed, probably.