They see themselves as first responders, police stand-ins.
Some news show did an episode on this concept...I can't remember which one, but the results weren't any surprise to anyone who takes a few minutes to think about this. They used a room arranged like a typical small lecture room and allowed people who thought they'd be better off if armed on campus to participate. They had people with a range of skill levels try it, including those who won awards as marksmen. The rest of the participants in the room were police playing the role of students panicking as a "shooter" entered the room.
Even in a controlled situation, where everyone knew there was no real harm from the "shooter," they all developed tunnel vision in their shooting. They took aim at the shooter, but were oblivious to other people running in front of them as people might if panicked, and didn't notice anyone on the other side of the "shooter." The simulation used paintballs as the ammunition, and there were more than just the shooter hit with the paintballs from the volunteers.
The other thing was that in some simulutions, they brought in a second shooter. Sometimes the second shooter was posing as another student "helping," only aiming at the first "shooter," and sometimes they were another criminal taking aim at other students. The volunteers couldn't tell the difference. Some weren't even aware a second shooter arrived, and those that were ignored them assuming they were helping not hurting.
The conclusion was simple and obvious...it's very different to be a good shot aiming at a paper target or while out hunting vs. when someone is shooting back at you. The people who want to play hero carrying guns around into classrooms generally lack the sort of training that law enforcement officers have to distinguish between the good guys and bad guys and maintain awareness of surroundings to avoid shooting other bystanders while confronting someone shooting back at them.
Edit: Here it is, it was on 20/20
https://www.youtube.com/watch?v=0MX3QtumSuE