If you want to change the factual situation, don't call that a failure of the analogy.

Obviously, if he were drawing back his arm to through a grenade, you could shoot him.

If he were drawing back his arm to pull the lanyard on an artillery piece aimed at you, you could shoot him.

If he were pulling back his arm to spring a trap door which would drop you 100 feet into a pit filled with spiders, snakes and scorpions, you could shoot him.

Maroon!

If he's pulling back his arm to throw a 36 inch punch at you while you are 25 feet away, you CAN'T shoot him.

Do I have to try, vainly, to keep you honest, too?

Arf (IF this guy's a CPA it would explain the high professional liability insurance. No, no chance!)