Atticus said that Bob Ewell spit in his face because Ewell needed "to have some kind of comeback." Ewell felt that Atticus had wronged him and in order to get his anger out, he spit in Atticus' face. Atticus doesn't react to it because a little spit in his face is better than Ewell going home and giving a beating to one of his children. Atticus has enough social status and respect from the community that allowing Ewell to get away with spitting in his face doesn't affect his reputation around town. This entire situation shows Atticus' patience with situations and his ability to stay calm and think clearly about consequences and the motive behind people's actions.
