In Chapter 11, what does Jem do and why?

On their way to meet Atticus after work, Scout and Jem have to pass by Mrs. Dubose's house. Mrs. Dubose is a very mean, sick old lady who sits on her front porch and yells insults at Jem and Scout as they pass by. The day after Jem's twelfth birthday, he and Scout go to town to spend some of his birthday money. On the way, Mrs. Dubose yells to Jem that he broke Miss Maudie's grape arbor that morning, which is untrue, and yells at Scout for wearing overalls. Then she starts yelling at them about how Atticus is defending "niggers," and says that Atticus is no better than "the trash he works for." Jem tries to follow Atticus's advice regarding Mrs. Dubose: just hold your head high and be a gentleman. In town, Jem buys himself a model steam engine and buys Scout a sparkly twirling baton she has had her eye on for some time.

On the way home, in a sudden fit of anger, Jem suddenly grabs Scout's baton, cuts off all the tops of Mrs. Dubose's camellia bushes, and then snaps her baton in half.