The idea is that Zimmerman placed himself in the immediate vicinity of Martin, where Martin became aware of the gun, causing him (Martin) to rightfully fight for his life. So a person may need to choose to believe this, or to believe Zimmerman's story.
It is absolutely possible that a jury will simply choose to believe Zimmerman, and reject all else. But the prosecution will try to make it seem that it is a rejection of hard facts to do so.
And while I don't think Zimmerman meets the standard definition of racist, I do believe he will have to rely upon having racists for jurors if he hopes to walk away from this.