Krugman looks at what led to the shooting in Arizona, I think this part explains it well:
As Clarence Dupnik, the sheriff responsible for dealing with the Arizona shootings, put it, it’s “the vitriolic rhetoric that we hear day in and day out from people in the radio business and some people in the TV business.” The vast majority of those who listen to that toxic rhetoric stop short of actual violence, but some, inevitably, cross that line
The problem with the hate filled rhetoric of Sarah Palin and Rush Limbaugh, or Glenn Beck, or perhaps Sharon Angle (wow, this list could get pretty long) is that even though most their followers are not going to commit mass murder, all it takes is one unhinged individual and you have a massacre like the one in Arizona.