First strike from the enraged perp. What might be expected next? Oh, gee! not popcorn, a fist to my 71 year old head.
While you might very well be worried about what he'll do next, you don't get to shoot someone unless they actually do something to present a deadly threat. In certain circumstances I might be worried about what a lot of people might do next, but I don't get to shoot them.
As you can clearly see, he was surrounded -- surrounded -- by empty seats. He could have moved to any of them. He could have demanded his money back and left the theater, if the manager would do nothing. But he chose to be a hard guy. Now he should do hard time.