1. Owning a gun is useless unless one has training in how to use it. Otherwise, your foot is at greater risk than the criminal. And a trigger happy vigilante would cause more trouble than it would prevent.
2. Accord to the Wall Street Journal, people at an Orthodox shul in Pittsburg on learning of the shooting responding by securing the doors and pulling out their own guns. Apparently, some people manage to have guns without ceremony. Indeed, since an armed guard would be the first and easiest target for a terrorist, any use of weapons should be done with a minimum of publicity.