Too much unknown information. As for the whole "he didn't have to confront him" him bit, it's kinda hard to remove those that are causing problems if they are never confronted and (imo) it is time for the citizens to quit relying on cops for every little thing. If someone sees suspicious behaviour it should be completely acceptable for them to investigate it should they choose (which means accepting all the risk that goes along with that) and not have to cower in fear and wait on police to "check it out." And if the person who's being approached decides they want to do something foolish simply because the person approaching them isn't a cop, well that is on them. Though I do think OCing in these types of situations would be a very good thing as it can help prevent a person from making a move that could be viewed as hostile (such as quickly reaching into a pocket after some heated words and in a manner that could be taken as reaching for a weapon).
Now with all that said, again there just isn't enough information to say with this case. I don't blame the watchman for approaching a "suspicious indvidual" in his neighborhood. But before even trying to say if I think the shooter was/wasn't justified I would need to know what all transpired prior to the shooting (exchange of words, actions of the dead person, why the shooter felt reasonably afraid of death/grave bodily harm, etc).