Except columbine had armed guards on the day of the massacre. Likewise, no mass shooting has ever been stopped by a "good guy" with a gun.
If you look at where mass shootings take place, you'll find that the vast majority take place in an area where guns are banned (or there's exactly one or two people in the entire building who HAVE a gun, in this case a typical school resource officer). I should also point out that the deputy at Columbine
wasn't wearing his glasses when he attempted to shoot, which is utterly baffling for anyone who plans on going anywhere with a loaded weapon. He also exchanged gunfire for a short time before stopping to call for backup, and allowed Harris and Klebold to continue their rampage 100% unimpeded.
I could go on about how you chose a biased source for your "NRA Myth of Arming the Good Guys" article, or how a number of the links in the article that I tried to use to find their sources simply lead back to OTHER articles on the site which has links that go to OTHER articles, or how some of the cases they picked (like the principal who was forced to run to his vehicle to retrieve his weapon during the shooting because he was banned from carrying it on school property) actually demonstrate a problem with their reasoning, or how some of the cases they give don't even count as mass shootings (shooting three or four specific people and walking out or committing suicide, which most people would just count as three counts of murder instead of lumping it in with Aurora), or how they used a case where the bad guy was armed with a semi-automatic rifle and body armor and thus couldn't be stopped by a man shooting him in the chest with a concealed handgun (which is not exactly an everyday situation for CCL holders), or their use of graphs that have their own in-house watermark instead of any citation for their numbers and have links that (again) lead to OTHER articles on their site.....but I'll let others look into that.
If you want an example of a mass killing that involved no gunfire at all, look at the
Akihabara Massacre in Japan. In a nation where firearms are under extremely tight scrutiny, a man killed 7 and injured 10 with nothing but a truck and a knife, both items that almost any of the mass shooters would have access to. According to the National Police Agency, 67 similar attacks have occurred between 1998 and 2007 (can you recall 67 attempted mass shootings in the United States in the past 10 years?). Exactly 7 years before Akihabara, a school janitor killed 8 children and seriously wounded 13 others and two teachers with a kitchen knife; the total number of casualties is only 3 lower than the Sandy Hook death count (excluding the shooter and his mother, which he murdered earlier). Sarin gas attack in Japan? 13 deaths, over
1100 injured, 50 of whom suffered long-term effects. A previous attack on the Tokyo Metro killed 8 and injured 144. The impure sarin used in the attack can be made by a chemist who simply has the right ingredients and recipe, and the method of attack is as simple as dropping a plastic bag full of the liquid on the floor, stabbing it a few times with your umbrella, and walking off.
But clearly, guns are the problem. Japan's certainly NEVER suffered massive amounts of casualties to killing sprees.