To say "fix the mentally ill" and we won't have so many massacres is simplistic. It is impossible to predict that a sane person with an AK-47 will always be so. It may also be simplistic to think that a person with an arsenal who is paranoid about the government is also sane.
Those who love their assault weapons and want to deflect criticism of such to the mentally ill need to remember that it was Ronald Reagan in the '80s who shut down a lot of state mental institutions and put a lot of ill people out on our streets. Now we hear the cry, "put them back in hospitals and treat them so we will be safer." Are you folks ready to pay higher taxes for that? I doubt it.
Can I make an insane suggestion? Let's outlaw assault weapons and greatly increase mental health care. Also, correct me if I'm wrong, but the vast majority of mentally ill persons are not violent, while many considered to be mentally healthy are.