How concealed carry can prevent mass shootings from occuring

In a recent discussion on FB revolving around "mass shootings", and how best to prevent them at a college, a friend of a friend made this statement about guns not being able to prevent crime"
"You can't stop a shooter who is already in range with a gun unless you shoot him first. If you have not drawn your weapon you cannot shoot him first. To suggest otherwise is disingenuous."

I have to disagree.

"Mass shooters" are generally people who have some issues, including power issues. They create a situation where they have all of the power - the power of life and death over others. Their goal is to exercise that power, even if it means that they do not survive the experience. Another goal seems to be the fame that they will gain if they rack up a high enough body count, and are shocking enough in their actions. This is why I do not use their names, since using their names rewards their behavior.

An armed person in a crowd of unarmed people has more power than the others. An armed person in a crowd that includes other armed people does not have more power than others in the crowd, even if they will have the advantage of initiative by being the one who starts acting first.

In most cases, "mass shooters" end their rampages as soon as they encounter an armed opponent. Not when they are shot or even shot at by an armed opponent, but when an armed opponent simply becomes part of the scenario. In some cases, they surrender, but more often, they kill themselves.

Here are a few examples:

The December 2012, Oregon shopping mall mass shooting stopped when the shooter saw an armed citizen. The CCW holder never fired, because he couldn't get a "clean shot" that would not endanger others. The shooter entered the "gun free zone" hoping to tally up a bunch of kills, and with a sense of power, because he was the ONE who was armed. The mere presence of an armed citizen changed that dynamic, and the shooter committed suicide. Police were still in their cars, on their way to the mall.

The December 2012, Sandy Hook Elementary school shooting also stopped when the shooter first encountered armed resistance. He shot himself when armed police officers arrived. Police didn't have to shoot him. Again, the dynamic of the situation changed, and he was no longer the ONE with the power.

Aurora Colorado, "Batman" shooter targets the only theater in town that is designated as a "gun free zone", surrenders to armed police as soon as they arrive. Again, the dynamic changed, and he was no longer the ONE with the power.

At Columbine, which is an unusual mass casualty attack, because there were two people involved, instead of the usual "loner". The perpetrators initially planned for it to be a bombing, but switched to shooting when the bombs failed. Even so, they killed themselves when the (armed) police arrived, changing the power dynamic of the situation.

At Virginia Tech, the shooter had plenty of ammunition left, yet chose to kill himself as the armed police closed in on him. The dynamic of the situation was changing, and he was not going to be the ONE with the power when they arrived.

The 2014 Isla Vista shooter was a bit different from the usual pattern, as he initially exchanged gunfire with police, but after he wrecked his car, and was no longer able to move freely, he shot himself as the armed police approached. In this case, loss of mobility was part of the changing dynamic.

In all of these instances but the last, the killers exclusively targeted people in "gun free zones" where they had a greater than normal chance of being the only armed person there. The gun didn't give them special power, being the only one with a gun gave them special power. If you are the only armed person, you can shoot again if you miss the first time. You don't have to worry very much about defending yourself. Others will run and cower - in many schools, teachers and students are taught to make themselves docile targets, rather than to "run, hide, or fight" (the DHS recommendation for workplaces).

Add a bunch of licensed concealed carriers (a group that includes off-duty cops), and the dynamic changes dramatically, making them less likely to strike.