It always seemed obvious to me that arming ordinary citizens in general would be a disaster, because some of the armed innocent people would mistakenly think they were in a Jared Loughner situation when they weren't and they'd accidentally shoot someone. The per capita mad gunman death rate is really small -- we're in a country of 300 million and it's not like these things are happening every hour somewhere. But how often does some ordinary citizen get firmly convinced he's in a Jared Loughner situation and he's got to kill a mad gunman? Don't know for sure, but in a country of 300 million, at least every hour seems like a good bet. If you saturate the public with concealed handguns, you get a lot of confused citizens killing other citizens whose behavior they misinterpreted.
The real horror hadn't occurred to me -- what if you iterate this scenario? Saletan:
In the chaos and pressure of the moment, you can shoot the wrong person. Or, by drawing your weapon, you can become the wrong person—a hero mistaken for a second gunman by another would-be hero with a gun. Bang, you're dead. Or worse, bang bang bang bang bang: a firefight among several armed, confused, and innocent people in a crowd.