Number one problem with mental health treatment (or lack of) in the US: No funding. Only treatment that is really funded is in prison. This is why police often charge mentally ill people with crimes in the hopes they can get treatment. But is that the best we will do for the mentally ill in...
Someone who is capable of this kind of shooting is probably a sociopath/psychopath. Read the literature, there is no effective treatment for them, they actually get worse with therapy. Read the what his friends and family said, he was described as "flat" and "remote", typical sociopath really...