At some point some people lose it all. We have seen it time and time again with world's dictators that they no longer even care what they are saying and how ridiculous they've become.
Take NRA's chief Wayne LaPierre and how he lost it all today. The criminals who go and commit these mass murder are not worried about an armed guard. If anything it'll only increase the violence as they'll seek more elaborate plans to kill the guard first.
Most of the guards will probably go to sleep after a week into the job while the assailant will walk past him not to mention in the midst of dealing with all the fiscal crisis let's spend billions of new dollars hiring armed guards around the country and create a police state.
There is a saying in Farsi; seek education from cradle to grave. Now we have; seek guns from cradle to grave.