When Atticus stands in Bob Ewell's shoes, why does he feel sorry for him?

In Chapter 23, Bob Ewell spits in Atticus' face while he is leaving the post office. Atticus calmly wipes the spit from his face and allows Bob Ewell to curse him out. That night, Jem tells his father that he is scared that Bob Ewell will attempt to hurt Atticus. Atticus explains why Bob spit in his face and feels sorry for Mr. Ewell. Atticus tells Jem that he destroyed Mr. Ewell's last shred of credibility and didn't blame him for spitting in his face. Atticus feels that if Bob spitting in his face saved Mayella one extra beating, he is glad. During the trial, Atticus described how Mayella broke the "unwritten code" of having feelings for a black man, and portrayed Bob Ewell as an abusive alcoholic. Since the trial, the Maycomb community has dismissed the Ewells. The entire county views Bob Ewell with contempt. Bob Ewell is a social outcast who is not respected in his home town. Atticus is able to perceive the difficult position Bob Ewell is in and takes pity on him.