From Chapter analysis of To Kill A Mockingbird (http://www.scribd.com/doc/38680600/Chapter-analysis-of-To-Kill-A-Mockingbird) --

Upon hearing that Jem was furious at Mrs. Dubose for 0frequently admonishing them, Atticus reminded Jem that Mrs. Dubose was old and ill, advising him to 'hold [his] head high and be a gentleman', adding that 'it's [his] job not to let her make [him] mad'. However, later in the chapter, when Jem and Scout pass by Mrs. Dubose's house, Mrs. Dubose made a snide remark, proclaiming that Atticus was 'no better than the niggers and trash he works for'. This provoked Jem so much so that he went up to Mrs. Dubose's front yard and
destroyed all her camellias, 'forgetting everything Atticus had said, forgetting that [Mrs. Dubose] packed a pistol under her shawl, forgetting that if she missed, Jessie probably wouldn't.