One example of this comes at Christmas during Atticus's visit to Finch's Landing. Scout is at odds with her obnoxious cousin Francis, who continues to bait Scout with taunts and other insults aimed at Atticus. After Francis deliberately provokes her when he "crooned softly, 'Nigger-lover...," Scout cannot restrain herself.
This time, I split my knuckle to the bone on his front teeth. My left impaired, I sailed in with my right... (Chapter 9)
Scout is not sure what "nigger-lover" means, but she knew it wasn't nice, and she refused to let her cousin "say somethin' about Atticus."
Scout had previously taken out her frustrations on Walter Cunningham Jr. by "rubbing his nose in the dirt" during lunch break on her first day of school. She had earlier been punished by Miss Caroline for trying to explain why Walter had no lunch or lunch money (the Cunninghams are poor), and Scout blamed Walter for her own problems with the teacher.