Why not always cast African Americans as antagonists?
Although the question is somewhat casual, constantly seeing an African American as the antagonist in movies can affect the way people see them. Some people may think of African Americans as only bad or horrible rather than looking at them as good or heroic.
Take the 5 min Take Look Different's Implicit Association Test and see the preference or bias you have toward certain races. (If you do have a bias, it doesn't necessarily mean that you're a bad or racist person.)
Did you take it? If so, think about how you got that bias, and if you don't have bias, great news! (Remember, if you have a bias, it doesn't necessarily mean that you're a bad or racist person.) Media such as TV can affect your bias. If people only watch a majority of TV shows or movies with a person of color as the antagonist and many white characters as the main protagonists, they may have some bad feelings toward the race. We need more diversity on screen.
No scenes of color?
Not casting African American actors/actresses at all can show that they are not worthy for a role. People also need to see more African Americans on scene because they need to see and be accustomed to the diversity of the world. The article, "Inequality in 700 Popular Films" by Dr. Stacey L. Smith stated that African Americans/people of color did not get featured at all in 17 films of the top 100 films of 2013 and 2014.
Smith, Dr. Stacey L. "Inequality in 700 Popular Films: Examining Portrayals of Gender, Race, & LGBT Status from 2007 to 2014." USC Annenberg. N.p., n.d. Web. 17 Feb. 2017.