it's common sense that if someone has already assaulted you, that the probability of a follow up assault is high.
Perhaps. But you have lots of options other than showing deadly force in response to popcorn being thrown at you, including two you've already identified, that makes the drawing of your weapon fucking bats.
since he initiated the violence, he would not be exercising LEGAL self defense. would he?
I bet he could make a plausible argument that he was. And, more importantly, I don't think it much matters to the people who just wanted to go see a fucking movie.