It sounds like they welcomed him into their church; then he left, came back, and shot them. Perhaps by welcoming him they tempted him off the Islamic path, into a state of fitna or even shirk, requiring him to kill them in defense of his eternal life and Islam.
Stopped by an Usher.
How many Sudanese are Muslim? @Curious2
http://edition.cnn.com/2017/09/24/us/tennessee-church-shooting/index.html