I knew walking out of that movie that people would take this very illicit, somewhat extremist rendition of slavery and allow it anger them towards contemporary whites. The N-word is used about 100 times, so I can empathize with those who don't readily understand the overall motif of this picture. Like Inglorius Basterds, it's a fiction depicting how ignorant society was, culminated by the oppressed's revenge.
Unfortunately, I doubt anyone who has this visceral reaction will look up the truth in that the African slave trade was started by non other than the Africans themselves.