People should not jump to conclusions with out all the evidence.
A couple of weeks ago, in the Daytona Beach shooting where an 82 year old man shot and killed a masked robber, attempting to enter his home. Some people, based on the original, very sketchy report, assumed that the 82 year old victim, fired blindly through a solid door, despite the fact that the Chief of Police, stated that the homeowner acted properly. They then proceeded to attack the home owner and the Chief of Police, based upon their false assumption of facts, and when later facts became available, including the video showing the door in question had a very large window in it, they refused to acknowledge the new information, and doubled down on their erroneous statements.
People, don't jump to conclusions without all the facts.
While, I agree that in general one should not shoot robbers simply because they are stealing, when you commit armed robbery, you are threatening people with a deadly weapon. With out being there at the scene and witnessing the event, we don't know how the perpetrators were behaving, They may well have been acting pretty crazy, which made the armed citizen decide he had to shoot to prevent them from killing people.
My general plan when confronted with a similar situation, is to get my gun ready to use, but not display it or intervene, and try to be a good witness. But if I believe that the robbers are about to commit violence, and that I can successfully intervene, then I will do so. To do less would be immoral. But I want to emphasize that I would not intervene unless I felt that not doing so would result in bodily harm or death to the victims or myself. Such decisions are often judgment calls, unless one waits until the perpetrators are shooting people, by which time it may be too late.
In this case, the Police have several witnesses to interview and possibly security camera tape. They will be far better informed about what happened than useless and baseless speculation from uninformed kibitzers.