@sp1814
I think the Dylan Roof case was not viewed as terrorism (despite the way it, possibly, had that effect) because it was not a 'political movement' (multiple participants required for that) and not a systematic attack on society (or segment thereof) but was a "one-off" event.
The US media has also loaded up the word "terrorist" with connotations along the lines of "attacking the American way of life", such that the term tends to bracket persons born outside the continental US. They do not wish to hurt their audience's brain cell by using the word terrorist and showing a picture of a white person simultaneously.
Technically, the 'loner' character trope is just as much an outsider, to society, as any foreigner. I'm not sure if that applied to Roof and he didn't 'hit back' at society, he targeted just one specific group for reasons of (I guess) pure prejudice. Not necessarily anti-religious, the church was just a known gathering place (sadly too many historical examples of that to mention).