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Imagine another darkened theater with an armed loon firing away. Now imagine a few armed "good guy" citizens blasting away at random, too. Deaths of innocents from friendly fire? You bet your ballistics.
The only true "good guy" with a gun is a trained officer of the law, and while they can't be everywhere an armed nutcase is, wounding or killing additional innocent civilians in a public area by untrained "good guys" would be kept to a minimum. Just don't tell that to the gun lobby.
(cartoons: top -- Jim Morin, Miami Times; bottom -- Darrin Bell, via Gocomics.com)