How does atticus explain mr. ewell's actions to jem?

"Jem, see if you can stand in Bob Ewell's shoes a minute. I destroyed his last shred of dignity at that trial, if he had any to begin with. The man had to have some kind of a comeback, his kind always does. So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that's something I'll gladly take."

Atticus goes on to say that Bob has gotten it all out of his system and there is nothing else to worry about, hoping to erase any fears the children may have. It was typical of Atticus to treat everyone fairly--even someone like Bob--and he wanted his children to understand why Bob made the threats he did.