These "How could this have been prevented" and "What warning signs were there that this guy would go off" angles are becoming tiresome and repetitive. In this particular case, the answer is basically "NOTHING, pretty much." It's a classic horse and barndoor scenario.

Society is f*cked up, and this is the new normal. Can you imagine trying to profile all these nutjobs BEFORE they have a chance to commit their hideous crimes?? You'd have to spend $$ BILLIONS to keep track of all the mentally unhinged people running around who might possibly potentially maybe probably empty their gun clips on a bunch of people someday.