Considering that this was against people at a Wednesday evening Bible study—as well as the fact that ABC to the contrary notwithstanding, South Carolina has a history of better race relations than almost any other American State; it seems far more likely that this was an act of hatred against people of Faith. (ABC in recent years has shown a clear antipathy to traditional religious Faiths. So, if I am right, they may well share some culpability for the atrocity.)
My guess it was not against people at a bible study per se . Those assembled were docile and certainly unarmed in church. There were enough gathered closely to allow quick accurate shooting to kill so many before they could either react aggressively or flee.
If you want to indiscriminately kill black people, an AME Zion Wednesday night prayer meeting is the place to go.