Why does Scout beat up Walter Cunningham?

In To Kill a Mockingbird, Scout beats up Walter Cunningham because she blames him for getting her punished on the first day of school.

The trouble starts at lunch, when Miss Caroline notices that Walter has no food and offers him a quarter. Walter refuses because the Cunninghams will not accept anything they cannot repay. Scout understands this Maycomb custom because Atticus once helped Walter’s father, who paid him back with food and firewood instead of money. Scout tries to explain, but she does it in a blunt, childlike way that makes Miss Caroline feel embarrassed and confused. Miss Caroline punishes Scout by bringing her to the front of the room and tapping her hand with a ruler.

Scout takes that humiliation out on Walter. After school, she finds him in the schoolyard and attacks him until Jem pulls her off. From Scout’s point of view, Walter caused her bad day: he had no lunch, refused the quarter, made Scout explain the Cunninghams’ poverty, and got her in trouble with the teacher.

The lunch that follows shows how misguided Scout’s reaction is. Jem invites Walter home, and Walter eats with the Finches. When Scout mocks him for pouring syrup over his food, Calpurnia scolds her: “There's some folks who don't eat like us, [...] but you ain't called on to contradict 'em at the table when they don't.” Scout is still learning that knowing about someone’s poverty does not give her permission to shame him.

The incident connects to one of To Kill a Mockingbird’s early lessons about empathy. Scout understands facts about Walter’s family, but she has not yet learned how to treat him with dignity.