I'm confused by the arguments saying, in effect, that someone who exclaimed, "Stupid ******!" after being knocked off a bike by a black person isn't necessarily a racist person. The mere fact that their first thought was a racial epithet indicates to me that said person is a racist. Maybe not a hate-filled white supremacist, but certainly a person whose worldview is informed by racial biases.
I can offer an explanation.
When I am really mad at someone - as in the circumstances you describe - I do not immediately become violent. Instead, I "use my words." In this case, the words I want are the most hurtful words that come to mind. Since I think racist slurs are hurtful, I might very well go down that road, not because of how I feel about someone's race, but because I want to create a certain, painful effect in my target.