In her drug-addled state, Mrs. Dubose had become filled with hate, even toward those--like Atticus--who loved her most. Unrepentantly racist and still living in the past, Mrs. Dubose despised Negroes, and she could think of no greater insult than by trying to compare other people with them. But she was particularly vicious during the scene with Jem and Scout in Chapter 11. Jem could deal with Mrs. Dubose calling them liars, and he was willing to let her insult about Scout "waiting on tables at the O. K. Cafe" pass. But when she turned her diatribe toward their father, "Jem stiffened."
"Not only a Finch waiting on tables but one in the courthouse lawing for niggers!"
This insult--"the shot"-- had hit close to home with Jem: Insulting him and Scout was one thing, but criticizing their father was different. As her mouth "trailed a long silver thread of saliva," Mrs. Dubose took one more shot at the children and their father.
"Your father's no better than the niggers and trash he works for!"
This time "Jem was scarlet," and he quickly decided to take revenge on the things Mrs. Dubose loved most: her camellias.