As of the writing of this post, nothing is known about the shooter or his motives, so as a counter weight to the inevitable lefty wails that he's a racist white guy, let me have a shot (so to speak).
1. He/She/It has lefty social/political views, and
2. He/She/It has a whole basket full of loose screws.
This is support of my view that if there were any way to keep lefty nutjobs from having guns, the instances of mass shootings would drop to near zero. I also don't see any practical way to do this, so I advise the rest of us to go armed.