Lest we forget

[Bramblethorn]

With the exception of the Tucson shooting, all these mass killings happened in gun free areas. In the CO shooting, the light was not all that bad, and the "armed bystander' could have fired at the killer's legs or head. Imperfect, but the end result might not have been as bad.

What's your source for "the light was not all that bad"? A theater during a movie showing is usually a pretty dark place.

Since he started by throwing a tear gas grenade, that adds smoke and badly-watering eyes to the mix, unless our hero is watching the film in a gas mask.

And of course, a lot of wannabe heroes freeze up when a real threat strikes, as demonstrated here: http://www.youtube.com/watch?v=uudWhXcPeh0&feature=youtu.be

People trained in tactical shooting will aim for center of visible mass by default - i.e. the torso, i.e. the vest. In bad light, confusion, and tear gas, they're probably going to have difficulty figuring out that he's wearing a ballistic vest, at least until they've fired a couple of less-than-effective shots and got his attention.

At that point, yeah, they could aim for the head. Smaller target, harder to hit, especially when he's moving... and as it happened, he was wearing a ballistic helmet, so that probably wouldn't have worked so well either. http://en.wikipedia.org/wiki/2012_Aurora_shooting

So when that doesn't work out, they might go for the legs. Harder target again - especially since in a cinema there are probably going to be seats blocking the view, but let's assume they get a clear shot down the aisle or something... oops, sorry, he was wearing bullet-resistant leggings too.

If our fantasy hero is an expert shooter and cool under fire, he's fired half a dozen shots by now - two at the torso, two at the heads, two at the legs. More likely, he's fired twice that number and emptied his clip, because people who aren't certain of their marksmanship tend to spray 'n' pray - see e.g. the Manhattan shootings of August last year, where police fired sixteen rounds at a single gunman, and managed to hit eight or nine bystanders in the process.

By this point, our hero is lying on the floor with a couple of 5.56mm rounds in him.

It's a fantasy. The guy who picks when and where the shooting is going to happen is at a HUGE advantage.

ETA: Are you sure about the Tucson shooting? According to this, the "armed bystander," whose name was Zamudio, arrived after the gunman had been subdued by unarmed bystanders. http://en.wikipedia.org/wiki/2011_Tucson_shooting

Yep, and by his own account Zamudio nearly shot one of those bystanders. http://www.slate.com/articles/health_and_science/human_nature/2011/01/friendly_firearms.html