If you have a lone gunman, first of all: where did they get a gun from? Having guns freely available to almost everyone, which appears to be the case in (some parts of) the USA, means that you already dramatically increase the number of people that will grab a gun in an emotional state instead of a knife or a baseball bat. So straight off the bat you have a worrying increase of unneccessary deadly violence.
That usually gets followed up by the argument that, if people really want to cause harm, they can get guns anywhere from the black market, no matter what the local legislation is. This may be true, but at least that means you have already eliminated a vast swathe of people who grab their gun out of an emotional outburst. So for the 'bad' people who are now left, we are talking extensive planning here, which usually means that even if you'd have a big armed troop of police at the site of the incident, they'd still manage to do quite a bit of damage before they get gunned down or detained.
Furthermore, replace trained police in the above situation with a bunch of often poorly trained civilians with guns, who will start shooting in response to the gunman. Suddenly you have a situation where we go from a single shooter which can hopefully be quickly localised and contained by the authorities to a mass shooting with bullets flying all over the place and lots more innocent bystanders potentially getting caught in the crossfire.
For every instance where the gunman gets taken down by a civilian without any other casualties, I can almost guarantee that there are several instances where the whole thing will turn into a bloodbath. It seems a popular thing in American (Western?) culture to champion vigilante justice. From 'Death Wish' to ' 'Batman', the lone heroic well-trained civilian armed to the teeth fighting crime that their corrupt local authorities do not stop is an image that we get shoved down our throats from childhood, but the reality doesn't work that way. Arming bystanders to avoid a rare (and it still is still relatively rare, despite it appearing a lot in the news these days) incident does not have a higher benefit return than having those bystanders misusing those guns or themselves going loopy and turning into perpetrators. In modern society, we have handed the responsibility of protecting innocents over to trained police and military forces so we do not have to do this ourselves.
To me, the above rationale is so obvious that it boggles my mind why the NRA and like-minded individuals are such a potent force in the USA. But I can only imagine that it stems from a very deep mistrust of your own local authorities and police. And if that is the case, I think it pays to think about whether that mistrust is validated or whether it is a result of years and years of Western culture (movies, comics, etc) indoctrinating you.
This post has been edited by Gorefest: 20 June 2016 - 11:50 AM