You can make the argument that if normal people armed with guns they can stop shooters but it doesn't really work with the situation in the Colorado movie theater.

1) The movie theater was dark for this occurred during a movie. Furthermore it happened in the first 15 minutes of the movie so people's eyes hadn't completely adjusted to reduced lighting. Thus it would have been hard to target the shooter and just as likely to shoot someone else.
2) The shooter used tear gas which besides being an irritant is another vision reducer.
3) The shooter was wearing a kelvar vest thus even if someone got a lucky shot on the shooter there is a good chance he wouldn't have gone down.

This was not a simple situation, adding more guns to the situation would not have made the situation more messy not less. This is not a convenience store with good visibility.