Dylan Roof was crazy. Crazy people latch onto any violent cause or trend in an attempt to make themselves feel rational and meaningful.
Most of what people his age have heard is divisive racial rhetoric...from the White House, the offices of various mayors across the country, and of course, from the academy. Most of them shrug it off and go on with their personal lives.
But I think it was pretty standard for a crazy person to latch onto it.
Which in past times he would have been considered under the control of demons.