People often automatically decide that a person is evil because they do evil or dangerous things to others. But in Harper Lee's To Kill a Mockingbird, it is shown that people who do evil or dangerous things do not entirely choose to be that way, and that they are victims of circumstance. This is most evident in the character of Mayella Ewell, whose actions lead to Tom Robinson's death, yet her actions are the only ones that make sense from her perspective. This is also demonstrated with Tim Johnson, a dog with rabies, who did not become a threat of his own choice but still had to be stopped before he could harm others. Finally, it is shown in Bob Ewell, a man with questionable moral character, but the blame for the kind of person he is is also on society and the way it has treated him. Mayella Ewell has the potential to do good, but instead does evil because her circumstances heavily restrain her. She testifies against Tom Robinson in court with her father despite Atticus making it evident that the story Mayella and Bob tell is a lie.
This damns Tom Robinson to a guilty verdict and eventually his death, because by testifying against Tom instead of telling the truth and saying he is innocent, she makes it the word of two white people against a black man, and in the racist setting of the story, "when it's a white man's word against a black man's, the white man always wins" (Lee 295). Causing an innocent young man to be convicted of a capital offense and killed is unquestionably an evil thing to do, but she did it because she was scared of her father. It is heavily implied that Bob abuses her physically and sexually, which Mayella almost admits in court when she says that as a father, Bob "does tollable, 'cept when—", until she sees her father looking at her hard, at which point she cuts herself off and recants her statement to just "he does tollable" (245). This moment shows that Atticus' inferences that Bob abuses her are correct, that she is afraid of Bob, and that she is afraid enough that she will not admit it to the courts. She will not admit it to the courts because in her mind, even though he is abusive, her father is the only person she needs to please. Maycomb looks down on her and stays away from her just because she is a Ewell, to the point where Mayella is so lonely that when Atticus asked her if she has any friends, "she seemed to not know what he meant, then she thought he was making fun of her" (256).
No matter what she does, Maycomb will definitely always scorn her anyway, so the only person she worries about pleasing is her father who may or may not treat her so badly at a given moment, even if that means hurting another person. In comparison, Tim Johnson had no intentions of hurting other people, but still became a threat that had to be put down. When Jem first sees Tim and brings him up, Scout describes him as a "bird dog, the pet of Maycomb" (122). Bird dogs are a type of hunting dog, helping their owner catch birds and other small game, which are the owner's food and livelihood. This means that Tim Johnson used to be a very helpful dog, but by the time he appears in the story, he has contracted rabies. He is no longer anything but a threat to others, but it is not his fault. As he is advancing up the street, Tim seems to be "motivated by an invisible force that was inching him toward" the people anticipating him, looking "more sick than anything" (126).
He did not choose to become sick, he was simply unfortunate enough to get infected, and he also did not choose to threaten others; it was the rabies that had ravaged his brain. And yet, while it was not his fault, he was still a threat that needed to be stopped. Atticus shoots him dead, even though Atticus had long ago decided "he wouldn't shoot till he had to" (130). Atticus had to shoot Tim Johnson, because while the dog had not wanted to be dangerous, he still was, and he needed to be stopped. And in that situation, the only way to stop him was to kill him.
Bob Ewell willingly does a lot of evil deeds in the story that harm or endanger others, but the fault is in society as much as it is in him. One of his worst actions is accusing Tom of raping Mayella, leading to him getting convicted and killed. When Bob saw Tom with Mayella, he saw him as "the best defense" to raping her (251) because he knew society would find a black man like Tom guilty immediately. But Bob is not wholly to blame for his own racism, because he grew up in a particularly racist family in a racist society.
When Atticus and his brother Jack are discussing the case in an earlier scene, Atticus asks him if he remembers the Ewells; Jack says yes and describes them, to which Atticus says that he is "a generation off. The present ones are the same, though" (117). This implies that Bob's father was also jobless, abusive, racist, and a drunkard. It is unfortunate that Bob became the man that he is, but if Atticus is right, then it is easy to see why he did. And if Bob and his father are the same, then surely Maycomb treated his father the same way they treat Bob. Bob's wife is dead, and according to Mayella, Bob has eight kids including herself, and all Maycomb gives them is "Christmas baskets, welfare money, and the back of its hand" (257), which does nothing to get them out of their poverty. With no means to pull themselves out of their poverty and ignorance, and no desire to, the Ewells' only hope is help from outside.
But Maycomb does not help the Ewells and lets them continue growing in poverty and abuse, and then looks down on them for being poor and abusive. In these three characters, it is shown that people who do evil or dangerous things are not evil people, they are still just people, who were unfortunate enough to be pushed by their circumstances to do bad things. Mayella Ewell shows that people do not choose to be evil on their own, but that their choice is influenced by fear and loneliness.
Tim Johnson shows that even when people who are a threat to others did not choose to be, they still must be stopped. And finally, Bob Ewell shows that the responsibility of improving the circumstances that cause people to do bad things falls on the rest of society, because people who do bad things can not do anything, and do not want to do anything, to improve their own situation. Understanding why criminals and morally corrupt people do bad things is important to society, because that is the first step to truly stopping evil deeds.
But understanding like that is hard, and many people take the easy way out, choosing instead to label them as evil, or monsters, or terrorists. People demand they be thrown into prison, and punished, and killed, and then they are shocked when more people do more evil things. That is exactly why being understanding is so important, and yet so few people do it.