You also need to take into account the other theory of the popcorn, which is that the popcorn flew into the shooter's face at the moment the bullet impacted the unarmed victim's chest and not before.
No, you need to take it into account.
It would make a difference, wouldn't it? If the popcorn was not thrown, then there is no physical assault and less reason to believe that self-defense was justified.
What if the victim put his hand into his pocket and said to the shooter, "I'm gonna cut your guts out, m------ f-----!". That's a verbal assault and might be enough to fear for one's safety.
Self-defense classes teach that an attacker with a knife can cover twenty-one feet before a potential victim can draw a gun and fire. You can't afford to wait until they are within ten feet to decide what to do.