How does Atticus' interaction with Bob Ewell prove the theme that it is important to treat others with dignity?
To say that Bob Ewell is treated with dignity, "the quality of being deserving of honor," may be a bit exaggerated; however, Atticus does speak to Bob Ewell politely and dispassionately--even respectfully at times--and after the trial he treats Ewell with Christian charity.
During the Tom Robinson trial, after Ewell is on the stand and tries to step down before cross-examination, Atticus addresses him as though he does merit respect by saying, "Just a minute, sir...." But because Atticus questions him and asks him to write his name, Ewell is suspicious of Atticus's motives. Later, when Atticus questions Mayella about what happened, her father is not so ignorant that he does not understand that the lawyer is attacking her credibility as well as his, as in the lengthy exchange of questions with Mayella. One question which probably raises Bob Ewell's ire is this one that Atticus asks Mayella:
"What did your father see in the window, the crime of rape or the best defense to it? Why don't you tell the truth, child, didn't Bob Ewell beat you up?"
After the trial, because Ewell realizes that he has been caught in his lie, and Mayella's credibility has also been challenged, he resents Atticus for the exposure before the community all gathered in the courtroom. So, he vows revenge upon Atticus Finch.
Mr. Bob Ewell stopped Atticus on the post office corner, spat in his face, and told him he'd get him if it took the rest of his life.
Atticus charitably "turns the other cheek" and does nothing but respond to Ewell's challenge, "Too proud to fight, you n****r-loving bastard?" by saying, "No, too old," and he walks away. Later when his son questions him about the incident, Atticus replies sedately that Jem should stand in Ewell's shoes and realize that his father
"...destroyed his last shred of credibility at that trial, if he had any to begin with. The man had to have some kind of comeback, his kind always does."