You are spinning yourself silly. You are claiming that throwing popcorn was assault - and therefore grounds for escalation to lethal force - yet state that a lesser, non-lethal action - brandishment - should have been avoided because it would have been a crime? I imagine the criminal penalty for brandishment is a LOT lower than that for murder 2.
That, quite frankly, is one of the dumbest lines of debate I have ever seen on FR.
It such a dumb idea that it is discussed at length in every self-defense class I have ever taken.
Throwing something in someone's face is the crime of assault. Throw popcorn in a cop's face and see if you go to jail.
Showing a deadly weapon without the justification to use it is a crime. Walk up to a cop with a gun in your hand and see if survive long enough to go to jail.
If it was not reasonable to shoot the popcorn thrower, then it would not have been reasonable to display a deadly weapon.
If the cop had brandished his weapon, would the popcorn thrower have been justified in drawing a gun, assuming he had one, and firing? If the ex-cop had ended up dead in that way, would you then be calling for conviction of the popcorn thrower for murder?