Why did Jem destroy Mrs. Dubose's flower garden?

One day, Scout and Jem walked by Mrs. Dubose's house. She was sitting on her front porch and she called to them. First, she accused them of playing hooky from school. Then she accused Jem of damaging Miss Maudie's scuppernong arbor. She also told Scout that she should be wearing a dress. Jem encouraged Scout to ignore Mrs. Dubose and go home.

Mrs. Dubose then called out again, talking about Atticus defending Tom Robinson. She...

One day, Scout and Jem walked by Mrs. Dubose's house. She was sitting on her front porch and she called to them. First, she accused them of playing hooky from school. Then she accused Jem of damaging Miss Maudie's scuppernong arbor. She also told Scout that she should be wearing a dress. Jem encouraged Scout to ignore Mrs. Dubose and go home.

Mrs. Dubose then called out again, talking about Atticus defending Tom Robinson. She used a derogatory term to describe Tom. Jem became enraged that anyone would say such a thing and speak of his father negatively. He stormed off.

Later, Jem walked down the street with Scout's baton. They walked by Mrs. Dubose's house. Then Jem did something that shocked Scout. It was something she knew Atticus would be appalled by:

We had just come to her gate when Jem snatched my baton and ran flailing wildly up the steps into Mrs. Dubose's front yard, forgetting everything Atticus had said, forgetting that she packed a pistol under her shawls, forgetting that if Mrs. Dubose missed, her girl Jessie probably wouldn't. He did not begin to calm down until he had cut the tops off every camellia bush Mrs. Dubose owned, until the ground was littered with green buds and leaves.