When most of the American population have access to guns, it takes all 300 million to behave themselves all day, every day, to have no gun deaths. It only takes one to flip out for a few minutes, and you have a massacre.
There are literally millions of people in the world with potentially dangerous mental issues. The problem is that 99.99% are functional, apparently normal people, and you wouldn't know them from any other law-abiding citizen. They are held in place by social pressures and the law. But you would do well not to trust them with a military style weapon.
America has to face the face that saying these people are mentally ill is not a solution, rather it is an indicator of how impossible it is to stop these massacres without some form of gun control.