7ronin
海軍少佐
Under the law self defense is not the only excuse. For example, if you saw someone about to kill another person you could reasonably use deadly force to prevent that murder. On the other hand, self defense may not be a valid defense in situations where deadly force would be an over reaction to the threat. Generally though you must be in imminent danger and in fear for your life or the life of another for deadly force to be legal. Often times the distinction is razor thin and the amount of time to decide negligible.