Following the Tom Robinson trial, Bob Ewell feels embarrassed and harbors a grudge against Atticus, who presented the ugly truth regarding his and Mayella's actions on the evening of November 21st. Bob Ewell vows to get revenge on Atticus for presumably ruining his reputation, which was never stellar in the first place, and even spits in his face when Atticus is leaving the post office.

In chapter 23, Bob approaches Atticus outside of the post office and threatens to kill him. Atticus refuses to fight Bob Ewell, who ends up spitting in his face and cursing at him. Bob Ewell's actions illustrate his despicable, ignorant nature and once again reveal his malevolent personality. Bob Ewell's actions also reveal that he is an angry, malicious person, who lacks respect and integrity. Unfortunately, Bob Ewell continues to seek revenge on Atticus and ends up attacking his defenseless children on their walk home from Maycomb's Halloween festival.

Bob Ewell is a cruel, abusive, ignorant man who drinks too much and doesn't take care of his kids. He accuses Tom Robinson of rape just because he saw Mayella through the window make a pass at Tom. He's so racist that he's willing to sacrifice Tom because he doesn't want anyone to know that his daughter would like a black man. This also reflects the racist atmosphere of the town.

After the trial, Bob is still angry at Atticus because Atticus shows everyone the true nature of Bob Ewell when he is on the stand. Bob wants revenge; he could care less what people think of him. Atticus makes him angry, and now Bob will get back at him because he is an evil man.

Bob Ewell is a spiteful man, and during the trial Atticus reveals the truth about him to everyone.  I don't believe he did it to hang onto any kind of reputation.  Bob Ewell didn't care what people thought - he was just plain evil intentioned.