I disagree. First, this is a vampire/horror/action/thriller, and the Grand Dragon of the KKK is a prime sort of villain to kill in such a story. And secondly, the twins -- for all their swagger -- had largely been defeated by white society, in the delta, in Chicago, and their "wins" involved getting out ahead of the Irish and Italian mobs.
Smoke being able to defeat the original evil at home was an important victory for him to claim in his final minutes of life and a way he could go be with his beloved and his child with his head raised high.