UPDATE: Interesting point raised by Glenn Reynolds. In response to a post by Volokh called "Do civilians with guns ever stop mass shootings?" he wrote, "Of course, if it’s stopped early, it’s never a mass shooting. . . ."
Turns out that the FBI definition requires four victims to achieve the "mass" standard. The average death toll when civilians stop it is 2.3. More, the author of that piece notes:
I found only one example of a shooter stopped by civilians who killed more than 3 people. Jared Loughner killed 6 people in Tucson, Arizona before he was tackled by two civilians. Maybe it’d have been less if one of those two men were armed.... If you compare the average deaths in a shooting rampage stopped by armed civilians to unarmed civilians you get 1.8 and 2.6, but that’s not nearly as significant as the difference between a proactive heroic civilian, and a cowering civilian who waits for police.So, do civilians stop mass killings? Almost never -- since when civilians stop them, they usually don't rise to the level of mass murder, and thus don't end up in the same statistical category.