What can be said about Bob Ewell's behaviour and character after the trial, when he spat on Atticus' face?
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.
I think there is a decided element of "saving face" on the part of Mr. Ewell. Consider that Ewell has been humiliated in court by Atticus. Ewell has been shown to be rather dimwitted and there are definite undercurrents that he sexually abuses his daughter Mayella. Although this jury will still take this man's word over somebody of colour's word, Atticus has clearly shown Ewell for the type of man he is, on display for all to see.