The actual investigators think you've jumped to the wrong conclusion:

“The suspect’s mother-in-law attended this church,” Freeman Martin, a spokesman for the Texas Department of Public Safety, said during a news conference Monday morning. “We know that he had made threatening texts and we can’t go into detail into that domestic situation that is continuing to be vetted and thoroughly investigated.”

“This was not racially motivated, it wasn’t over religious beliefs, it was a domestic situation going on,” Mr. Martin added.

https://www.nytimes.com/2017/11/06/us/texas-shooting-church.html