Right now, I'm waiting to make a decision on this topic until more details are released.
I don't know what was said by either man during the arguments. It has been stated at least that the guy shot, Oulson, did "attack" first by throwing popcorn at Reeves. If Reeves can clearly see it's a bag of popcorn that was being thrown at him, then he has no defense. If he fired after being hit by a bag of popcorn, he has no defense. A bag of popcorn is not a credible threat to cause grave bodily harm or death. It just isn't.
However, if Reeves shot while Oulson was in the middle of throwing the bag and he was unable to ascertain the action made by Oulson was to throw a bag of popcorn, then he could at that moment in time be justified. Which is what the law hinges on at the moment the action is taken. If Reeves shoots after being hit with popcorn, then the shooting is certainly not justified at all.
Also, it can depends upon what was said between the two men. More specifically what Oulson may have said before, during, and or right after throwing the popcorn. If he said something like, "I'm going to kick the shit out of you old man" and reach to throw his popcorn at Reeves and Reeves pull his gun and shot while Oulson was throwing it is justified shooting. Sucks, but justified.
What does my intuition tell me though? I do not believe this to be a justified shooting. I think Reeves shot after the popcorn was thrown and impacted. Which by that point anyone would be able to tell a bag of thrown popcorn is not a credible threat that can cause grave bodily harm. Which if that is the case may Reeves rot in prison.