Texas passed a concealed handgun law because a man in 1991 drove his truck through the wall of a Luby's restaurant in Killeen, Texas, and proceeded to open fire. One of the customers in the restaurant, who survived, testified to the Texas Legislature that she had a gun in her car, but couldn't bring it into Luby's; had she been able to, she argued, she could have stopped the gunman and saved lives.
Well: maybe not.
Scroll down and take the time to watch these ABC videos from 2009. They are quite instructive. Take note that the Texas law requires 10 hours of training, answering a test of 50 questions, and then firing 50 rounds. After that, you're as ready as any of the subjects in that video.
Interestingly, guns still aren't that prevalent in Texas, because no one has pulled a gun and stopped a shooter. The last shooting I know of (other than front-door assassinations, and unless you're going to start answering the front door with a loaded gun....) only involved the shooter having a gun, but enough damage was done there that more guns simply would have meant more chaos (he wasn't even trying to kill anyone around him; he was trying to shoot a guy he never hit). The other mass shooting was at Fort Hood (also in Killeen, Texas). It took several armed police officers to stop that one.