"A quick Google search failed to find any authoritative statement that either the law or the community demanded that he not be armed. Can you provide a source?"

A number of the early news stories that I read on Yahoo mentioned that according to the rules of the neighbourhood watch group to which he belonged, he was not supposed to be armed. A couple of reports said that Z.'s carrying of a gun had been a source of some contention within the neighbourhood watch. Whether those stories are still available on line or not, I have no idea. I did not save the pertinent urls (if that's what they're called).