He is not a cop, he is a citizen.
If I shot and killed someone for throwing popcorn at me, I’d be in jail for life too.
That is correct as far as it goes but is misleading because it ignores a few very important facts. Fact, he is a retired cop which means he has been through many years of repetitive training on how to react to violence from all sources and in all situations. Fact, he was promoted within the police department which indicates his ability to make proper decisions at the proper time as dictated by that training. Do you really think he can unlearn those years of training? Where is that stop button located? Is he given an uninstall program at his retirement ceremony?
Without a doubt this turned out totally wrong for all involved but there are many things here that are not yet knows or discussed.