In chapter 23, we see that Tom Robinson has been found guilty and is being sent to another prison, while his appeal is going through. Atticus proved that Tom was innocent, and that makes Bob Ewell angry. When Bob threatens Atticus, Atticus just takes it and doesn't do anything in retaliation. Atticus thinks that Bob is doing this, because Atticus made him look like a fool on the witness stand, so he thinks once Bob gets it out of his system, it will all end.
Atticus is well known for the kind of person that doesn't take things to personally. He tries to see the reason why people act the way they are. He tries to see the good in a person. By his reasoning, Bob is just angry at the way Atticus made him look. Atticus is happy to take the brunt of his anger, instead of Bob taking his anger out on anyone else. Jem and Scout are still worried about what Bob might do, but Atticus tries to reassure them.
Atticus doesn't realize that Bob is about to take his anger out in the worst way. Atticus is about to be faced with a parents worst nightmare. Bob Ewell is not the kind of man to let things go. Bob is determined to get his revenge on Atticus one way or the other. He realizes that Atticus is not going to try to fight him, so he goes after the one that matters the most to Atticus. What Bob doesn't realize is that Jem and Scout have a very special person watching over them.
"'I wish Bob Ewell wouldn't chew tobacco,' was all Atticus said about it" (217).
Jem and Scout describe their father as dry, but Atticus' comment about Bob Ewell is almost unresponsive. Nothing Bob Ewell says or does can rile Atticus up. In fact, nothing ever riles Atticus up to the point where he loses his self-control, self-respect, or dignity. On the other hand, some might say that Atticus was naive to think that Bob Ewell wouldn't do more than just threaten him. For example, Atticus is truly shocked when he discovers that Mr. Ewell actually tried to kill his kids with a knife in chapter 28. When Scout describes the attack to Heck Tate in chapter 29, Atticus interjects with the following:
"I can't conceive of a man who'd. . . I thought he got it all out of him the day he threatened me. Even if he hadn't, I thought he'd come after me" (269).
This passage shows that even though Atticus barely responded to Bob Ewell's threat at the time, he underestimated the depth of Ewell's intention, motivation, and ambition. At the time Bob Ewell threatened him, Atticus climbed into Bob Ewell's skin, but didn't understand as well as he thought he did. Little did Atticus know that had he really climbed into Ewell's skin, he would have seen rage and revenge.
When Mr. Ewell threatens and spits on Atticus at the post office, Atticus just takes it humbly and peacefully walks away, saying he's not too proud to fight; just too old. When Jem fears for Atticus's safety, he later tells Jem to put himself in Mr. Ewell's shoes. Having made Mr. Ewell look like a fool at the trial, he knew Mr. Ewell would have to at least make some comment or do something in retaliation. Atticus explains that if Mr. Ewell takes out his frustrations on himself (Atticus) rather than on Mayella or one of the other kids, he'll take it every time. Throughout the novel, Atticus consistently acts unselfishly, always thinking of others. This is why he does not seem offended, ashamed or angry at Mr. Ewell's insults.