The real question is why is everyone at a military base unarmed?
The second question is that since there are about 2.2 million people in the US with this diagnosis (http://www.schizophrenia.com/szfacts.htm) it is clear that the overwhelming majority of these cases do NOT commit these multiple homicides, so no action is really necessary.
Barring the above - if you’re a danger to yourself or others, you should be locked up. When locked up, you are unarmed. You should not be let out until you are NOT a danger to yourself or others, therefore, you could have a weapon.
So no legislation barring schizos from having weapons is needed.
Even if implemented, nothing stops them from getting knives from doing the same, like happens in china (google it).
Or driving in to a crowd of folks somewhere with an assault SUV.
The real question is why is everyone at a military base unarmed?
That was a decision made by Snopes Clinton, who hated the military, the Second Amendment, and all us commoners who believe it means exactly what it says. Take it from there.
To answer your question, Bill Clinton issued an executive order to that effect in, I believe, 1996.