The school's armed guard didn't stop the perpetrator. Rather than precipitating a gun fight in a crowded classroom, a courageous teacher talked him down, and he surrendered.
I've constantly heard the argument on Twitter and iRL that the answer to gun violence is more guns. If a violent person is aware a target is armed, it will act as a deterrent. Of course, this assumes disturbed people with guns are thinking rationally.
Fortunately, many conservatarians also stress the need for improved mental health. How people in a country with a utopian "limited government" and no insurance can get treatment is a deafeningly unanswered question. Ron Paul would probably be able to provide a meek answer, but it would take him a long time.
Accessible mental health care and education coupled with crisis training for people in allegedly high risk occupations would solve far more than simply arming everyone. It will take more work though.