I think there are plenty of societies which would not have responded to this incident with violent force, and would have checked themselves before they wrecked

I think there are plenty of societies which would not have responded to this incident with violent force, and would have checked themselves before they wrecked themselves as a consequence of normal, functioning, day-to-day job responsibilities. The officers failed because the event horizon for each and every one of them was set way, way too high. Before you assume the caller was right, you verify. This thing of "arming for the absolute worst that could happen" is a direct consequence of the rest of society, being allowed to actually procure and apply the weapons of mayhem and crime, to other "innocent citizens". A kid died tonight. It was because there was a tool of violence in the hands of another kid.