No motive established yet. Although his In-laws went to that church. The parents and grandparents of the wife and child he abused back a couple of years ago, that got him kicked out of the USAF on a bad conduct discharge. It also prevented him from obtaining his Texas carry permit. Hence he purchase of a Ruger AR-15 rifle.
The thing is his in-laws weren't in that church that day, but he didn't know it, or so it is believed by law enforcement.