What is the prejudice shown by Mr. Radley's assumptions about the disturbance in his backyard?

In Chapter 6 of To Kill a Mockingbird Scout, Jem, and Dill want to get a look at the legendary Boo Radley. So one night, the last night of the summer vacation, they sneak under the wire fence on the Radley property. It's Dill's last night in Maycomb, so this will be his last chance to get a good look at the mysterious Boo. Suddenly, someone comes out of the house and fires a shotgun. The children make a run for it. Unfortunately, Jem gets his pants snagged on the wire fence and has to take them off in order to escape.
The shot was fired by Nathan Radley, Boo's brother. He tells his neighbors that he "shot at a Negro in his collared patch." This shows us Radley's prejudice, one shared by most of the townsfolk of Maycomb. He automatically assumes that if someone's trying to steal something from him, then they must be African-American. Radley confirms his prejudice by stating that the next time it happens, he'll aim low, whether it's at a dog or an African-American. The statement reveals that Radley places African-Americans on the same level as animals, giving us an insight into just how prejudiced he really is.