Arming everyone to the teeth isn't really much better than taking guns away from everyone. If everyone is armed, you simply can't make the argument that it's any more difficult for some wacko to get hold of a gun and look to commit violence with it.
The idea that someone else will have gun and take him down quickly in order to limit the damage stands to a degree. Many people don't want to own guns and don't know how to use a gun. If you're relying on someone who doesn't want the gun or doesn't know how to properly use the gun to act, he/she runs the risk of making a situation worse, and on the extreme side, turns our streets and public facilities into the wild west.
It boils down to education. You have to take courses and tests in order to get a driver's license because a car wields massive killing power. Why not something similar for gun ownership? You need to take a gun operation, safety and ownership class and then pass a test for a gun license just like you would have a driver's license.
It boils down to a willingness to kill. We have always been able to kill, but we built societies where killing was culturally dissuaded.
Then the 60's came and our culture war, where societal norms were destroyed and the most base was celebrated. The left has attacked the notions of restraint, responsibility, and particularly integrity - now we wonder why the lowest elements in society have no restraint and lack the integrity not to kill others.
Free love, do your own thing, never judge the choices of others, there is no right or wrong - just what is right for you. These are the lessons of the last 50 years, last Friday is another example of the results. Oh, and this is exactly as planned, the culture war was designed to create mayhem and cause this society to crumble. It's working like a charm.