Many themes in Edgar Allen Poe's works concern mankind's internal battle between good and evil. Similarly, in To Kill a Mockingbird, Harper Lee shows us that human beings have both good and evil natures. Therefore, one might conclude the theme shared in common between Lee's work and Poe's works is that all of mankind faces an internal battle between good and evil.
The short story "William Wilson" is one of Poe's many works that depicts the internal battle between good and evil. In the story, the protagonist, going by the pseudonym William Wilson, meets a boy in school who looks exactly like him, shares the same birthday, and shares the same school enrollment date, as well as many other similarities. The only difference is that the narrator is prone towards immoral behavior, whereas his nemesis behaves very morally. As the narrator progresses through life, each time he is about to commit one of his worst acts, his nemesis appears out of nowhere to thwart him. The actions in the story represent the protagonist's battle with his inner conscience, symbolized by his nemesis. Similarly, in the short story "The Black Cat," the narrator's alcoholism transforms him from a gentle, tender-hearted person into a lunatic responsible for abusing and eventually savagely murdering his beloved pet cat and wife. The narrator is relaying his story from his cell, his execution date soon approaching, with a dual purpose. First, in typical Poe fashion, the narrator wants to relay what he feels is a real-life horror story.
Second, he wants to make a confession, which we see when he says such things as, "I blush to confess it." The fact that he is bearing his soul in confession shows us he has a conscience, a conscience that lost a battle against his alcoholism. Hence, even in "The Black Cat," Poe shows us that mankind is engaged in a battle between good and evil, albeit to Poe it is a losing battle.
In To Kill Mockingbird, the scene containing the lynch mob led by Walter Cunningham Sr. is one clear moment author Lee uses to portray the internal battle between good and evil human natures. Scout is shocked that Mr. Cunningham would lead a lynch mob and threaten Atticus's safety, because she has been led to believe he is a good person, as she expresses when she asks Atticus the next day, "I thought Mr. Cunningham was a friend of ours. You told me a long time ago he was." Atticus's reply shows us his belief that all people have their good and bad sides, a belief that serves to develop Lee's theme concerning the internal battle between good and bad human natures:
Mr. Cunningham's basically a good man ... he just has his bind spots along with the rest of us. (Ch. 16)
In other words, according to Atticus, Mr. Cunningham battled with his bad nature, which manifested in his violent racial hatred, even though he is "basically a good [person]," just as William Wilson and the narrator in "The Black Cat" battled against their consciences.