It has already happened repeatedly it has no effect on the anti self defense fanatics. 2.5 million times violent crimes are stopped with legally used guns. Even when mass murders are stopped it gets such little media attention the public is unaware that it even happened.
February 12, 2007, In Ogden Utah: Kenneth K. Hammond carrying a concealed weapon stopped a massacre at the Trolley Square Mall in Utah
December 11, 2007, In Colorado Springs Colorado: After killing two people at a mission training school the killer drove 65 miles killing two more people in the parking lot before entering New Life Church carrying a rifle, two pistols and a backpack holding more than 1,000 rounds of ammunition. intent on killing 1,000 members inside the church. Which he would have done had Jeanne Assam been there with her legally carried handgun. Jeanne Assam a member of the congregation (not a security guard as the media erroneously reported) stopped the killer before he could begin his massacre
January 16, 2002, In Grundy, Virginia: Tracy Bridges & Mikael Gross stopped a mass murder at Appalachian School of Law, after lives were lost while they retrieved their guns from their cars, where they were required by policy to keep them. ( If college students legally licensed to carry concealed weapons been able to carry gun on school property no innocents would have died)
1998, In Edinboro, Pennsylvania: Restaurant's owner, James Strand stopped a killers rampage in Edinboro, Pennsylvania saving the lives of 240 students at a school dance a full eleven minutes before police arrived. One teacher John Gillette, 48, was killed
October 1997, In Pearl City Mississippi: Principal Joel Myrick stopped a slaughter at Pearl High School with his .45 after lives were lost while he retrieved his .45 from his car, where it was required by law to be kept. (Had legally licensed school principles and teachers been able to carry his gun on school property none would have died)