As a few have said on this thread multiple times, including myself, there was Zero Justification for this guy to pull out a firearm and start shooting. It is sad and pathetic that the discussion would proceed any further than that. The fact the victim had any mental disability should be irrelevant. The fact the shooter's occupation was policeman is irrelevant as he was off-duty and not in uniform. If anything, the fact he works as a policeman daily, should normally give some assurance that he would act in a professional and responsible manner while off-duty and on. And these days, the opposite is more and more common.
He was in a public place, and there were no weapons visible or has it even been suggested the victims presented any weapons during the encounter. It comes down to the fact the shooter felt that since he had a firearm on him, he had every right to use it to defend himself and his family, regardless of the situation. There were several options available, the most obvious of which would have been simply notifying store security, or even better....discussing the situation on the off chance that perhaps there was an explanation. But even if there wasn't, and we're talking about a person wanting to pick a fight with another person, pulling out a gun and starting to fire at every target in your immediate vicinity, is nothing less than attempted murder, as well as murder in this case.