Boo Radley is a symbolic mockingbird in the novel and is a victim of the prejudice that results from his unorthodox, reclusive lifestyle. In chapter one, Scout elaborates on the many false rumors surrounding Boo Radley, saying,
People said he [Boo] went out at night when the moon was down, and peeped in windows. When people’s azaleas froze in a cold snap, it was because he had breathed on them. Any stealthy small crimes committed in Maycomb were his work (Lee, 9).
Boo is unfairly blamed for any "small crimes" simply because the citizens find his reclusive nature strange and menacing.
Boo also has a bad reputation among the local children and is unfairly perceived as a "malevolent phantom" who goes out of his way to harm children. Jem demonstrates his prejudice towards Arthur "Boo" Radley by telling Dill,
Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that’s why his hands were bloodstained—if you ate an animal raw, you could never wash the blood off. There was a long jagged scar that ran across his face; what teeth he had were yellow and rotten; his eyes popped, and he drooled most of the time (Lee, 13).
Jem's fantastical description of Boo Radley is extremely inaccurate and a result of the negative rumors he's heard about Boo from various community members, particularly Miss Stephanie Crawford.
Later on, Walter Cunningham Jr. walks home with the Finch children and explains to Jem that he almost died by accidentally eating one of Boo Radley's poisoned pecans. He tells Jem,
Almost died first year I come to school and et them pecans—folks say he [Boo] pizened ‘em and put ’em over on the school side of the fence (Lee, 24).
Walter's comments illustrate his prejudice and paint Boo as a nefarious man who enjoys harming innocent children.