The point is that a vast majority of murderers aren't highly determined. That's why in every other Western country, the same people end up assaulting or stabbing one or two people, rather than mowing down a school.
There's also the fun fact that arming people to the teeth doesn't do very much to prevent despotic governments as well.
When the stars threw down their spears
And watered heaven with their tears:
Did he smile his work to see?
Did he who made the lamb make thee?