There is stupid and then there is STUPID. But I am not sure that I understand your criticism. The intention of the shooter was just to get the guy’s license plate and wait for the police. He WAS trying to observe the situation until the police arrived. When the belligerent idiot noticed, he got out of his minivan and followed him into the store, pushing and shoving the entire time.
To me your arm chair assessment indicates only that you probably have not been in a similar situation. Because of my former profession I have been in the path of idiots on many occasions and it is often not easy or even possible to avoid a serious confrontation.
The police in this area have a pretty fast response time. I do not know when a 911 call was placed and the article states that the armed citizen was getting the license plate to give to the police. It does not say the police had been called.
Maybe you like armed citizens playing cops but I don't. As to my "armed chair assessment" I have witnessed and been involved in a number of situation like this and there has never been a shooting. Just observe and let the cops handle it.