What explanation does Atticus give for Bob Ewell's personal attack and threat?

From the text:

“He meant it when he said it,” said Atticus. “Jem, see if you can stand in Bob Ewell’s shoes a minute. I 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. So if spitting in my face and threatening me saved Mayella Ewell one extra beating, that’s something I’ll gladly take. He had to take it out on somebody and I’d rather it be me than that houseful of children out there. You understand?”