Those are all easy arguments to counter.
At columbine, the SRO engaged the shooters at a range of around 70 yards. Hitting a human sized target at 70 yards with a handgun is pretty difficult, regardless of who you are, and when you add in the fact it was a dynamic shoot, with both SRO and target moving, and target shooting back, it would have been impossible. After the shooters moved into the building, the SRO followed SOP at the time and waited outside for backup. Once the shooters realized they were caught, they killed themselves.
At VT, the campus police never confronted the shooter, until the very end. And when he was confronted, he ended the spree by killing himself (a seemingly common thing that happens in these things).
I could easily argue that in both these cases that intervention by good guys with guns prevented more people from being killed, and that had they been confronted sooner (instead of SOP at the time being wait for backup), more people could have been saved.
And you are correct about the military base being a gun free zone. People have a misconception that military bases are full of armed men and women. The truth is that the only people armed on a military base are the MPs and contracted security.
At the Oregon mall shooting that occured before Sandy Hook, evidence points to an armed man drawing on the shooter, but not firing because he wasn't sure of his backstop (he saw people running around behind the shooter). Nevertheless, it appears the shooter saw this good guy with a gun, because within seconds of seeing the armed good guy, he moved to the final location where he placed a bullet firmly into his own brain. The mere presence of a good guy with a gun, very well could have saved dozens of lives.
I know, I'm preaching to the choir here.
You are, however, correct. There is no way to confirm whether a good guy with a gun will always stop a bad guy with a gun every time. However, without a good guy with a gun, a bad guy with a gun will have free reign to kill indiscriminately. In the end, there are only three ways these mass shootings will stop:
1) The crazed madman of a shooter, for some reason, decides to stop
2) The crazed madman of a shooter runs out of ammo
2) A good guy with a gun makes them stop.
EDIT: I just wanted to add to Glenn's post. We call police, EMTs and fire fighters "first responders." In reality, they are second responders. We, the public are first responders. It would be irresponsible of me to say, "I won't use this fire extinguisher to put out this fire, because we have professionals that can do that." Of course, the difference is, I am here, they are somewhere else. Armed police, no matter what, have a response time. Having worked for TSA in an airport that had no airport police, I have first hand experience at response time. Let's just say that if a gunman had decided to shoot up the lobby of the airport, he would have been able to have free reign for at least 5 minutes. A lot of damage can be done.
VT, Columbine, Ft. Hood, and many others suffer from the same problem. The real first responders have been neutered of their ability to respond effectively. They have to wait on the response of the second responders.