You simply do not whip out a gun and fire a bullet into someone because they are arguing with you over texting, and they make a move toward your popcorn. You also don't do so because they threw popcorn in your face.
You simply do not. That's not a civilized society at that point.
The situation they were in, the context, has to be considered. If you get into an argument with someone in a darkened alley at night, with nobody else around.. and the other person is younger, larger, and getting increasingly agitated - then I could potentially see someone, particularly an older man who is more frail, erring on the side of caution when that person makes any sort of move toward them, and firing in self-defense.
But this wasn't that situation. These two were each with their wives, they were in a crowded theater with numerous pairs of eyes on them. Management knew there was an issue. There was light, whether from the screen or aisle lights or whatever. Even if texter guy was throwing a punch, you take the punch - you don't end a life. Because if it's a one punch kill, you aren't stopping it period. If he's intending to get on top of you and beat you, there are plenty of people around to pry him off.
None of this is going to be something you can really actively THINK about in those moments, of course, but the point is that this is just not the type of setting or environment which should have you on that kind of DEFCON 5, hair-trigger kind of alert. You should not be in a "shoot him instantly when he moves toward me in any way, and figure out what he was doing afterward" type of mindset, in that type of place.