In an attempt to gain some power in a shabby, pitiful existence, Mayella costs a man his life. Ironically, when Atticus finally shows Mayella the respect she so craves, she accuses him of making fun of her and ultimately refuses to answer his questions.
Bob Ewell would also like to improve his family's station, but the fact that "he was the only man [Scout] ever heard of who was fired from the WPA for laziness" proves that he isn't willing to earn it. Ewell is a drunkard and an abuser who is despised throughout the community, and very likely by his own family. But in accusing Tom Robinson, he sees what he believes is a brass ring. In his mind, the town should think him a hero for saving Maycomb's white women from a "dangerous" black man. Defending his daughter by going to court should raise his family's stature. If they don't gain more respect from the community, at least Bob won't have to live with talk in the black community about a white woman making a play for a married black man. Unfortunately, all of Ewell's plans backfire. By the end of the trial, he and his daughter are proven liars, he's been publicly identified as a sexually and physically abusive father who fails to provide for his family, and the entire town knows that Mayella made sexual overtures toward Tom. Instead of improving his life, Ewell cements his family's horrible reputation once and for all.
In this situation, Bob Ewell can do little but try to recover his own pride. He makes good on his threats to harm the people who embarrassed him in court. He rejoices in Tom's death. Bob Ewell is the kind of person who actually seems to enjoy being despicable.
