Why is Atticus not afraid of Mr. Ewell?

Jem and Scout are certainly afraid of Bob Ewell after they hear about the incident at the post office. It is soon "all over town" that Bob Ewell has threatened to kill Atticus after spitting in his face; Atticus has responded in a manner so "dry" and casual that it seems he is completely unafraid. This is not to say that he really is entirely unafraid—after all, he must have been startled. However, it seems that he does not entirely believe Bob Ewell to be serious in his threats.
Atticus's "peaceful reaction" is sufficient enough that Ewell backs off and does not actually initiate a fight; therefore, Atticus may be justified in believing that Ewell's bark is worse than his bite, as it were. Atticus seems to think that Ewell has a very bad attitude but will not actually take steps to enact his threat.
Atticus explains to his children that Bob Ewell may have meant his threat "when he said it," but he doesn't think he means it in general. Rather, Atticus thinks that by making this threat to Atticus, Ewell has excised anger which otherwise might have been taken out on his family—that is, that it is now out of his system.

Atticus does not fear Bob Ewell for several reasons. Atticus initially felt that Bob Ewell took out all of his aggression towards him when he spit in his face at the post office. In chapter 23, Atticus tells Jem and Scout that he does not blame Bob Ewell for spitting in his face and sympathizes with the man. Also, Atticus does not fear Bob Ewell, because Atticus sees the best in people and does not comprehend the fact that Bob is wicked enough to attempt to murder his children. Following Bob Ewell's attack, Atticus is astonished and cannot wrap his mind around the fact that someone would attempt to harm innocent children. In chapter 29, Atticus tells Sheriff Tate, "He [Bob] was out of his mind . . . I can’t conceive of a man who’d—" (Lee, 273). Overall, Atticus does not fear Bob Ewell because he feels that Bob already got his revenge by spitting in his face and does not believe that Bob is evil enough to harm his children.