What it means (without having read it but based upon other things I've read) is that white people excuse white mass murderers are mentally ill and are not predisposed to murder just because they are white. Meanwhile, white people do not think black people who murder are mentally ill; instead, white people believe that's just the way black people are because they are savages. So white people cannot be blamed for all the shootings; just the shooter. Conversely, black people are all blamed because white people believe that every black person is capable of snapping due to some hard-wired inferior emotional control due to their race.