This is faulty logic my friend, for a few reasons:
1. People are polite at the gun range because, for the most part, people who go to licensed gun ranges are those of us who are repectful of firearms and the people around us...not everyone (in fact generally a very small % of owners) is as respectful or goes to the licensed ranges...I've had several instances out at some no-monitored outdoor shooting sites of having to deal with some extremely disrespectful and dangerous owners.
2. Many of the people who commit these acts don't look at the world with the same logic as you or I... for several, I doubt they would consider retaliation before they did these things.
3. There are a lot of owners in public who posess firearms, yet lack the proper training to use them in a crisis scenario. Without the proper training to identify the threat, target and fire accurately, and consider surroundings and possible collateral damage, arming the general populace may turn out to be more dangerous than the inital gunman in certain situations.
In the end, it's not nearly so simple as "Give everyone a gun"; without the proper respect and skills to make that owner a responsible gun owner, it does nothing to create a truly acceptable solution.