On the Wednesday night of June 17, 2015, two days before Juneteenth, a day when the abolition of slavery in the United States is celebrated, Dylann Roof sat in a historically black church before knowingly and willingly choosing to murder and maim at least nine people. On that Wednesday night, Dylann Roof took action on thoughts and plans he had harbored for many months beforehand. On that Wednesday night, Dylann Roof chose to commit an act of terrorism and chose to become a terrorist.
While all terrorism is crime, not all crime is terrorism. Roof clearly committed the crimes of murder and aggravated assault, but because his crimes were politically motivated they were also acts of terrorism. This is what separates terrorism from traditional crime; the violence of the act is designed for an audience with the hope that it will incite political change. In this case, Roof was hoping that his act would incite a new civil war - pinning blacks against whites. In addition, his target couldn't have been any less hardened. You really can't get a softer and less prepared target than a bunch of Wednesday-night church regulars; no doubt an unwitting and defenseless civilian target.
It has been mentioned and debated in the media as to whether Roof is mentally ill, and whether or not this played a part in his actions. He likely is, but it is also of no significance since many criminals demonstrate symptoms of mental illness and it rarely excuses the perpetrator. Having a mental illness would be more of a debatable factor if there wasn't an available understanding of his motivation; such as in the active shooter mass homicides in Aurora, CO, and Sandy Hook, CT. Roof's motivation was racism.
Lastly, other than the fact that Roof chose to kill black people in a Christian church, there is no other evidence that he harbored ill-will toward Christians as a whole. If his motivation was a religious gripe against Christians, he could have carried out his terrorism at a number of churches much closer to his home and that had a more diverse parishioner base. It has become clear that Dylann Roof wanted to kill blacks - regardless of their religious affiliation - and that his form of terrorism was to be a racial one.