I've mentioned before that if someone walks into a school building with a gun and opens fire, there are two possible outcomes: 1) The person continues to shoot until he/she finishes, then is captured, killed, or commits suicide; 2) Someone inside the school intervenes. There is a third possibility, but it remains theoretical -- that the police arrive quickly enough to stop the shooter in mid-havoc.
Anyone who believes that the third possibility may become a reality anytime soon believes in the tooth fairy.
The second possibility presupposes a person already on the premises who has the wherewithal to intervene. That ponderous sentence can be translated "someone with a gun." The hierarchy of my university believe in the tooth fairy. Gun toters believe in their marksmanship. The police acknowledge that the only real way to stop a shooter is with an on-site shooter. However, they say that it would be difficult in such a situation for the police to know who is a good guy and who is a bad guy.
This is a reason for not carrying a gun? For one thing, it'll be all over by the time the cops get there, so there is no reason to worry. For another, anyone who has a license to carry a weapon knows the procedure for after a gunfight -- put up your gun and put up your hands. For a third, the police should have a procedure in place -- shoot anyone with a gun.