Why did Marissa help Melba when she was attacked by the man in the field?

On her way home one day, Melba is savagely attacked by an irate white man who tries to rape her. He's angry at the landmark decision of the US Supreme Court in Brown v. Board of Education (1954), which ruled that school segregation was unconstitutional. Fortunately, Marissa is one hand to help. She hits the man over the head with her book bag and tells Melba to run.
Marissa's actions are somewhat surprising. At school, she has a deserved reputation as a bully. In fact, she bullies Melba on a regular basis, forcing her to give up food and money—despite her father being a well-to-do minister. But here she shows no hesitation in saving Melba from a terrifying ordeal. No explicit reasons are given, but it's more than likely that Marissa intervened because she didn't want to see someone from her own community become the victim of an act of racial and sexual aggression.