Civilians with guns make the whole situation more dangerous and much harder for police to control. This has been known for years.

Imagine a gunman on the loose in a shopping centre...police are called in. Now if you have "good guys" running around trying to find /kill the shooter(s) how do the police know who is who?? All it does is increase the chances of friendly fire and also the real gunman getting away.
It has also been shown that civilians who do have a firearm in this situation will run or endanger more people by trying to be the hero.