why does scout fight with Cecil Jacobs?

Cecil Jacobs, teases Scout, saying that her father "defends niggers". Scout will not accept insults about her father and fights Cecil. Later, she asks Atticus what the phrase means, and he explains that he has decided to defend a black man named Tom Robinson, who lives in a settlement behind the town dump. Atticus says many of the town people think he ought not defend Tom because he is black. Scout asks why he's still doing it if people don't want him to, and Atticus responds that if he didn't take the case, he wouldn't be able to "hold up my head in town," represent his county in the legislature, or even tell his children what to do.