Why does Mr Radley think that he was shooting at a "negro"in his garden? What does this tell us about the society in which they live?

Mr. Radley assumed that only a black man would be thieving in his garden.... this tells us the racist mindset that pervaded in Maycomb. Personally, I think Mr. Radley's words to Miss Stepahine allude to the fact he knew his thief....

“No,” said Miss Stephanie. “Shot in the air. Scared him pale, though. Says if anybody sees a white nigger around, that’s the one. Says he’s got the other barrel waitin‘ for the next sound he hears in that patch, an’ next time he won’t aim high, be it dog, nigger, or—Jem Finch!”