It's a common pro-gun argument that "only a good guy with a gun can stop a bad guy with a gun". It's not true. At the recent shooting at Seattle University, the shooter was taken down by an unarmed man while he reloaded. When Senator Giffords was shot in Arizona, the shooter was tackled by a man who a holstered gun. Instead of shooting irresponsibly, the man made a judgement that his body would be more effective. You do not need a gun to be the hero. Having a gun doesn't make a person more heroic. It just increases that person's potential to accidentally shoot someone.
If I see a random civilian carrying a gun, I have no way of knowing if they are with the shooter or what qualifies them to use that gun. The last thing I want is to add another lethal weapon to a crisis.
http://nymag.com/daily/intelligencer/2013/11/nypd-police-shootings-numbers-2012.html
Looking solely at information from the NYPD in 2012, 14 of the 30 people that were shot by the police were accidentally hit. Even our trained officers have roughly a 50% rating, and we are trying to put our trust in Joe Schmo who has no qualifications or training. Joe might be a world-class competitive shooter, or he might have just bought his first gun last week because the NRA told him Obama was going to ban them if he did rush out at buy one. Since I don't know where on the spectrum Joe Schmo is, I'll take my chances at diffusing the situation. A second gun will only escalate it.