What about all the massacres that were prevented by armed citizens?

The Atlantic chronicles some instances where armed citizens stopped or mitigated massacres...

Many of the worst American massacres end not in the capture of the gunman but in his suicide. In the 2007 mass shooting at Virginia Tech, for instance, the gunman, Seung-Hui Cho, killed himself as the police were set to capture him. But in other cases, massacres were stopped early by the intervention of armed civilians, or off-duty or retired police officers who happened to be nearby.

In 1997, a disturbed high-school student named Luke Woodham stabbed his mother and then shot and killed two people at Pearl High School in Pearl, Mississippi. He then began driving toward a nearby junior high to continue his shooting spree, but the assistant principal of the high school, Joel Myrick, aimed a pistol he kept in his truck at Woodham, causing him to veer off the road. Myrick then put his pistol to Woodham’s neck and disarmed him. On January 16, 2002, a disgruntled former student at the Appalachian School of Law in Grundy, Virginia, had killed three people, including the school’s dean, when two students, both off-duty law-enforcement officers, retrieved their weapons and pointed them at the shooter, who ended his killing spree and surrendered. In December 2007, a man armed with a semiautomatic rifle and two pistols entered the New Life Church in Colorado Springs and killed two teenage girls before a church member, Jeanne Assam—a former Minneapolis police officer and a volunteer church security guard—shot and wounded the gunman, who then killed himself.

It is still unclear if the Connecticut school shooter's mother worked at the school. She is not listed on the roster of teachers. She may have been a substitute in the past, but that is unclear at the moment. So, after killing mom at home, why did this monster drive her car to an elementary school? We may never know for sure, but looking at the locations of other recent mass murders, one thing stands out. They were mostly 'gun free' zones such as schools, malls, theaters and universities. In other words, there was the availability of a lot of soft targets. Another thing these mass murders have in common seems to be troubled youth with a history of mental health issues. Instead of more gun control, we need to reconsider how we as a society deal with juvenile offenders and the mentally ill.