It had nothing to do with mixed couples, "Abel" just hated white people.
SPOILERS.
I know that this movie is loosely based on a real life case which iv'e seen in one of those American documentary series, but the movie is actually nothing like the real life case.
It seems that the movie storyline is about a black man who dislikes mixed couples, that isn't the issue at all, if you notice all the snide comments from "Abel" he is always saying something Anti-Caucasian, he only actually mentions them as a couple and their different races two or three times.
The scene in the bar when "Abel" mentions his wife going somewhere in the car with her "white boss" is more proof that he disliked white people in general. There was no proof that his wife was having an affair, her travelling in the car with her boss could have been for any innocent reason. And even if she was having an affair, what does the man's race have to do with it? The race of the person would only be an issue if you were racist which "Abel" clearly was.
There wasn't enough emotional connection from Samuel L. Jackson's character towards them to show that he disliked them as a mixed race couple. At the beginning when he looked shocked that Kerry Washington was with the white guy - doesn't mean that "Abel" was racist against mixed couples, it means he thought that the older black man was her husband (which was actually her father) and was shocked to find out that he wasn't, that was all.
"Abel" had a problem with white people in general especially white men, that other comment that "Abel" made in the bar is also a confirmation "It means you white boys think you can take whatever you want, and i hate it! That's what it means!" - that comment from "Abel" shows that he clearly detests white people.