What did Jem do when Mrs. Dubose said Atticus "lawed for niggers"?
In Chapter 11, Jem and Scout decide to head to town to spend some of Jem's birthday money. As they pass Mrs. Dubose's house on their way to town, she begins verbally attacking Jem and Scout in her typical manner. After accusing both children of playing hooky on a Saturday, she wrongly blames Jem for breaking down Maudie's scuppernong arbor earlier in the morning. Mrs. Dubose then directs her attention toward Scout by telling her that she'll be waiting tables at the O.K. Cafe if she doesn't change her ways. (Lee 135) Her next comment hits home and makes Jem stiffen when she says, "Not only a Finch waiting on tables but one in the courthouse lawing for niggers!" (Lee 135)
Following the hateful comment by Mrs. Dubose, Jem's demeanor changes. He turns "scarlet red," becomes silent, and displays no expression of happiness when purchasing his toy steam engine in town. On the walk back, Jem takes Scout's new baton and begins to smash Mrs. Dubose's camellia bush. After destroying the camellia bush, he snaps Scout's baton over his knee in a fit of rage. Scout's explanation for Jem's reaction is that "he simply went mad." (Lee 136)
Jem had long endured the negative comments and personal attacks from Mrs. Dubose until she verbally attacked his father. Jem looked up to his father more than anyone in his life, and that insult stung Jem deeply. When Atticus found out about Jem's rampage he made Jem apologize to Mrs. Dubose who requested that Jem read to her as his punishment.
You wrote that this is in Chapter 10. It is actually in Chapter 11. You can find it there.
Jem and Scout are walking down the street when Mrs. Dubose, as usual, is criticizing them. She talks bad about their whole family. What really gets Jem is that Mrs. Dubose says that their dad is working for the black people.
After this, Jem and Scout go to the store and buy their toys. Scout's is a baton. On the way back, Jem takes the baton from Scout and uses it to destroy all of Mrs. Dubose's camellia bushes.