During breakfast the night after the mob shows up at the jail, Atticus compares the men with “a gang of wild animals”. One of the reasons he does this is to highlight the mentality of a mob. When you think of a pack of animals, you think of them acting as one organism. Think about a pack of wolves bringing down an elk or a pride of lions attacking a zebra. They have one purpose, and work together for that purpose. The mob of men came to the jail with one thing in mind - to harm Tom Robinson, and they weren’t going to let anyone get in their way, not even Atticus, who is a respected member of the community. But Atticus also highlights the fact that a mob is still made up of people, and that by connecting with just one person the way Scout did with Mr. Cunningham, a mob can be stopped. He helps the children see that even though a mob is a scary thing, it can be thwarted by reminding those “animals” of their humanness.

Lee, Harper. To Kill a Mockingbird. (Soft Cover). Perennial, 1961

In Chapter 16, Atticus explains mob mentality to his children while they are eating breakfast. Atticus begins by excusing Mr. Cunningham's actions by saying that he is still a good man but has "blind spots" like everyone else. Atticus proceeds to tell the children that individuals act differently when they are part of a mob. Atticus tells his children that men in a mob tend to lose sight of their individuality and are capable of doing things they would never typically do on their own. Atticus then compares the Old Sarum bunch to a "gang of wild animals". Atticus refers to the mob of men as a "gang of wild animals" because their mentality and violent nature was dangerous and unrestrained, like savage animals. Atticus understands that a mob of men is capable of committing extremely violent crimes, which is why it was so surprising that Jem, Scout, and Dill's presence prevented the Old Sarum bunch from harming Atticus and lynching Tom Robinson.

Atticus compares these men to wild animals because of their fear of those who aren't like them and their willingness to put the constraints of the law aside and threaten him with "pack" violence.