The "smoking gun" here is indeed mental health. But that problem is so difficult to control that one can only fall back on the implement used, which can cause rapid deaths in short order. Violent people are generally not easily deterred. If it's not guns they can use, they'll find ways to create makeshift explosives.
What still gets me is how reactive people are. Nobody got chills down their spine seeing these massive assault weapons easily purchased by the general public? Apparently not enough... have to wait until someone misuses them. Just confounds the dickens out of me.