What constitutes a "terrorist" attack? Does it have to be specifically planned by a particular ideological group or a "state sponsor of terrorism?" Apparently not. Nidal Malik Hasan's rampage at Ft. Hood is generally thought of as "lone wolf" terrorism; and even though IS took responsibility for those two inept jerks who tried to attack the Texas cartoon contest, it seems a lot more likely they acted on their own.
To just about everybody but Fox News, it is clear that Dylann Roof's attack on Emanuel African Methodist Episcopal Church was an act of terrorism, inspired, at least in part, by things he found on the internet. According to the Southern Poverty Law Center, most of the terror attacks in the USofA are inspired by far-right militias, neo-nazis, and the "sovereign citizen" movement. Islamists have some catching up to do.
If you can find another 21-year-old who even can find Rhodesia (now Zimbabwe) on a map, much less identify with an apartheid government which was overthrown well before he was born, you've found a very unusual 21-year-old. Yes, Dylann Roof was inspired by good old, down home South Carolin racism, but it seems indisputable that his ideology was shaped by the internet, which is overflowing with racist militia, neo-nazi, and sovereign citizen crap. The kid is a terrorist — and he's not alone out there.