“Scenario as described has the assailant walking away”
No it doesn’t. Read it again.
If you can’t safely get into your vehicle, lock the doors and remove yourself from the situation, then use of lethal force becomes appropriate and defensible as you are in fear for your life. Being a dumb@ brandishing and firing, giving into pride, fear and emotion, is exactly the wrong thing to allow yourself to do. As I said, you’d better keep your wits about you, otherwise it’s you who is going to jail.