Atticus is the type of gentleman who believes the best in people, and also hopes for the best. When it comes to the Cunningham lynch mob, he was lucky because things could have turned out much differently had the mob been insanely bent on hanging Tom Robinson. As it is, Atticus has the chance to discuss his beliefs with Jem after the mob incident. Jem doesn't understand how Atticus could still call Mr. Cunningham a friend after what he did with the mob the night before. Jem believes that given the chance, Walter Cunningham would have hurt Atticus and that's not friendly. Atticus responds as follows:
"Mr. Cunningham's basically a good man. . . he just has his blind spots along with the rest of us. . . He might have hurt me a little. . . but son. . . a mob's always made up of people, no matter what. Mr. Cunningham was part of a mob last night, but he was still a man" (157).
The belief here is that mobs are created by human beings who have feelings and families just like Atticus. He marvels, though, that it took a little girl to break up the "wild animals;" and, he even suggests that maybe children should police the town in order to remind the grown ups of innocence and what's really important in life. Again, Atticus and the children were really fortunate not to have been placed in front of an angry and determined lynch mob; otherwise, they would have simply removed anyone standing in their way and taken Tom without listening to reason. It's a good thing that Atticus was right about Mr. Cunningham being "just a man" in this case.
Concerning the group of men who had come to take Tom Robinson from the jail, Atticus explains to Jem that although a mob is a thing, it is made up of people--of individual men. The lynch mob arrived as a group and they intended to kill Tom as a group, but all it took was for one individual member to remember that he was a human being, and the group dissolved. Scout's innocent talk with Walter Cunningham reminded him that they were both human, and despite "his blind spots," Cunningham separated himself from the "gang of wild animals." Once Cunningham, the leader of the group, had decided against taking these actions in front of children, the rest of the men followed his lead.