Scout fights with Cecil Jacobs because he insults her father for defending Tom Robinson.
Cecil Jacobs is a town kid, “who lived at the far end of our street next door to the post office.” Scout gets into a fight with him when he insults her father. Scout has promised Atticus that she will not fight, and he said he would “wear her out” (spank her) if she did it. Yet Cecil angered her so much she forgot.
Cecil Jacobs made me forget. He had announced in the schoolyard the day before that Scout Finch's daddy defended niggers. I denied it, but told Jem. (ch 9)
Cecil Jacobs was telling the truth. Scout does not understand why what he is doing is a bad thing.
Atticus explains that all he is doing is defending a Negro, Tom Robinson. He tells her that there is nothing wrong with that. He believes that he must do it, because he could not hold his head up if he did not. He has to do it for his own self-respect, because he has to do what he thinks is right.
In a way, both Scout and Atticus are standing up for what they believe in. Atticus is defending with words, and Scout is defending with fists. Atticus is trying to show her that his way is better.
At the beginning of Chapter 9, Cecil Jacobs announces to the schoolyard that "Scout Finch's daddy defended niggers." Upon hearing this, Scout clenches her fists and tells him to take it back. It is not directly stated whether Scout actually fights Cecil on the schoolyard. Lee writes that Scout was prepared to fight and forgot about her father's advice, but never mentions if Scout actually takes a swing at Cecil. Later that night, Scout has a conversation with her father about Cecil Jacobs's comments. Atticus explains to his daughter that he is defending a black man named Tom Robinson. Atticus then encourages Scout to keep her fists down and control her anger. The next day on the playground, Cecil Jacobs refuses to take back his comments and says his parents think Atticus is a disgrace. Despite Cecil's derogatory comments, Scout maintains her composure and calmly walks away.