Why is Bob Ewell the way he is in To Kill a Mockingbird?Please consider his past circumstances and his family, in particular Mayella.
In many of their narratives, both Anton Chekhov and Guy de Maupassant have written about the peasants of their countries, peasants with whom they were well acquaintained. Common to these Russian and Norman peasants is a contorted perspective, almost perversely petty and myopic. An observation similar to that of viewing the peasants of Chekhov and Maupassant is made by Scout in Chapter 17:
Every town the size of Maycomb had families like the Ewells. No economic fluctuations changed their status--people like the Ewells lived as guest of the county in prosperity as well as in the depression. No....public officer could free them from congenital defects, various worms, and the diseases indigenous to filthy surroundings.
Like Luster Sexton of Erskine Caldwell's Tobacco Road, the peasant of Georgia whose family has lived on the same plot of land for generations, and like the other peasants of Russia and Normandy who, also, have resided in the same places, there is a certain mentality that comes of Bob Ewell's congenital defects and his family history of living on the lowest level of society. For some reason, these people have no ambitions, but feel an envious resentment toward anyone they perceive as a rival or better than they. In their pettiness, they seek to pull down or harm the other person, and by doing so, they thereby somehow feel better about themselves. This is why Bob Ewell insists on Mayella's claiming that Tom Robinson has raped her; he must, at least, be somehow superior to a Negro. And, this is why he spits in Atticus Finch's face at the courthouse and later tries to kill his children.
People like Luster Sexton and many of Faulkner's characters and Bob Ewell are the stereotypical "poor white trash" whose ancestors were probably indentured servants and the like from the lower levels of the United Kingdom. At the very least, Bob Ewell and his ilk are the least ambitious elements of the white southern society, whose location in town certainly perpetuated the ignorance of this type.
There is even a remark that Nathaniel Hawthorne makes in his novel, The Scarlet Letter, that explains the stereotype that the character Bob Ewell represents. Hawthorne observes,
Human nature will not flourish, any more than a potato, if it be planted and replanted, for too long a series of generations, in the same worn-out soil.