On Wednesday night, a gunman—identified as 21-year-old Dylann Roof of Columbia, South Carolina—murdered nine people at a weekly prayer meeting at Emanuel African Methodist Episcopal Church in Charleston. Among the dead is 41-year-old Rev. Clementa Pinckney, pastor at the church and a four-term Democratic state senator. The shooter is a young white man, and police are calling the murders—the worst mass shooting in South Carolina’s history—a hate crime. It’s appropriate...