It is clear from the first time Bob Ewell answers a question in court that he blames the rest of the world for his misfortune. In chapter 17, when asked if he is Mayella's father, he responds that if he isn't, he can't do anything about it, because Mayella's mother is dead. Chapter 17 also tells us that he was "too distracted" by Mayella to be able to run after Tom Robinson. The biggest indication of who Bob Ewell blames comes at the end of chapter 17 when he says,
"Tricking lawyers like Atticus Finch took advantage of him all the time with their tricking ways".
In short, Bob Ewell blames everyone for his misfortune.
Harper Lee's To Kill a Mockingbird tells the coming-of-age story of a young girl, Jean Louise "Scout" Finch, and her brother, Jem, during a time when institutionalized racism was pervasive across the American South. Lee was a product of Alabama, and the fictional town of Maycomb, Alabama is set there. The story also takes place during the Great Depression, when already low-income communities were victimized even more. In To Kill a Mockingbird, the lowest of the low is Robert E. Lee "Bob" Ewell, patriarch of a desperately poor family, the eldest child of which, Mayella, is at the center of the rape trial that forms one of the centers of Lee's narrative.
Throughout Lee's novel, it is made very clear that Bob Ewell and his family are the personification of poor white trash. So beyond the pale does this family exist that the rest of Maycomb's citizenry collectively turns a blind eye to the Ewell's extremes. Early in the novel, Scout, the story's narrator, describes her father, Atticus', view of the Ewells as follows:
"[Atticus] said that the Ewells were members of an exclusive society made up of Ewells. In certain circumstances the common folk judiciously allowed them certain privileges by the simple method of becoming blind to some of the Ewells’ activities. They didn’t have to go to school, for one thing. Another thing, Mr. Bob Ewell, Burris’s father, was permitted to hunt and trap out of season."
In addition to the town's acquiescence in the Ewells' exclusion from mainstream society, Lee repeatedly references that family's dependence on welfare for its survival, as in the following passage:
"The tribe of which Burris Ewell and his brethren consisted had lived on the same plot of earth behind the Maycomb dump, and had thrived on county welfare money for three generations."
It is with this privileged status in mind that one assesses Bob Ewell's defensiveness regarding what could be called proper society. Atticus Finch, in Ewell's estimation, represents that society and the way it looks down upon them. The only real quote in Lee's text that illuminates this perspective occurs during the rape trial of Tom Robinson:
“You’re left-handed, Mr. Ewell,” said Judge Taylor. Mr. Ewell turned angrily to the judge and said he didn’t see what his being left-handed had to do with it, that he was a Christ-fearing man and Atticus Finch was taking advantage of him. Tricking lawyers like Atticus Finch took advantage of him all the time with their tricking ways.
Bob Ewell's sense of victimization is further reflected in his daughter's response to Atticus' questioning of her on the witness stand. Mayella, having observed Atticus's cross-examination of her father, is on the defensive regarding the lawyer's questioning of her, angrily asking, “You makin‘ fun o’me agin, Mr. Finch?”
Bob Ewell uses his sense of victimization as a means of justifying his own disgraceful treatment of his children and of anybody who crosses his path. By viewing himself as a victim of the more educated classes, he is able to justify his anti-social behavior and his isolation from the rest of society.