The following segment from Comedy Central's The Daily Show tries (sometimes successfully) to be funny but also makes some serious points about the notion that the only thing that stops a bad guy with a gun is a good guy with a gun. (Caution: Some naughty cable TV language ahead.)
The guy in this video might be deliberately throwing the exercise at the end, but real-world tests tend to go the same way for people with limited training. Here's a famous example from a 2012 ABC News report:
In real life in the U.S., the FBI says that armed civilians do succeed in stopping a bad guy with a gun -- in about three percent of shootings -- but more often the bad guys are stopped by actual police officers or by unarmed civilians. Armed civilians without training far too often shoot themselves or friends or family members (sometimes intentionally).
My friend Bruce Arthurs wrote an excellent post on the Aurora theater shooting that's worth reading. His point is that had an audience member been armed the death toll would likely have been higher. Even worse would be a bunch of good guys with guns, strangers to each other, each mistaking the others for additional terrorists. Give guns to enough people and someone wanting to kill a bunch of people wouldn't need a weapon; it would be enough to point at someone and yell, "Look out! He's a terrorist with a gun!"
I'm a military veteran who used to carry a sidearm, and when I was doing it actively I was a pretty good shot with a handgun. I'm not in favor of banning guns, which politically just isn't going to happen anyway, no matter how much those who sell guns try to convince their more gullible customers otherwise.
But it does seem pretty obvious to me that encouraging every random, poorly trained (or untrained) civilian to carry guns anywhere they like to the terror of the public isn't a smart idea. If you think the Second Amendment guarantees this as a right, you need to do a little reading about its history. A good start would be the first four words of the Amendment itself, which are "A well regulated Militia." Not "A bunch of unregulated assholes with guns."