How does Atticus explain Bob Ewell's actions to Jem?

From the text:

"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."

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