Banning guns in the us is impractical and would be wholly ineffective. While there are numerous convincing arguments supporting this position, all you need to realize is that we are likely only years away from desktop 3d printing of the rifles and their modifications, magazines, etc.
Is a fantasy. Even worse, attacks like this are essentially impossible to prevent. The guywas blocks away in a hotel room. Bottom line is that when someone snaps like this, there is going to be the potential for massive carnage.
Only thing to do is somehow reach these people beforehand. Good luck.
And clearly something about the modern United States culture is causing these at at an alarming pace, it's worth thinking about what that cause is. But it's clearly not access to guns because that correlation doesn't hold up anywhere else in the world.