Stand your ground laws aside - the standard for using lethal force is that it is only justified when you are confronted with imminent, otherwise unavoidable threat of death or grave harm.
The trick is have a base of knowledge which allows you to make that determination sooner.
You are judged on what you know at that time, and then if your judgment is reasonable to those reviewing the facts of the case.
What is reasonable? Depends on the person's basis of knowledge and background.
Person A "just knows" he's going to be attached.
Person B has taken instruction on pre-assault cues and that information is solidly grounded on statistics, and police-criminal interactions of the instructor sharing the material with the students. He knows he is going to be attached.
Person C "just knows" he's going to be attached...because he spend 8 years of college as a bouncer in 14 different bars during the time it took for him to get his masters in women's studies (Don't laugh, the ladies thought he was sensitive and he had a good time...).
Both A, B & C respond in exactly the same way to the same situation.
Who is having an easier time explaining why he put someone in the hospital?