There are many examples in To Kill a Mockingbird of the three things you mentioned--good, evil, and social injustice. Most of the actions of Bob Ewell are of an evil nature; Atticus, in particular, is nearly always good. If you are asking about an example where a combination of the three is evident, here is one example:
When Jem explains to Dill about the nature of Hot Steams, Scout denounces him by claiming that "Cal says that's nigger talk." Scout displays all of these examples in one breath. (1) She is attempts in good faith to tell Dill that Hot Steams are only myths; (2) she inadvertently--for she is too young to understand--spouts an evil, racial epithet; (3) and, her use of the word "nigger" shows a social injustice. The fact that Calpurnia is quoted additionally shows that she believes such foolish talk is indicative of her own people.
The trial of Tom Robinson and the ensuing courtroom scenes as well as the scenes afterwards clearly portray the co-existence of good and evil as well as social inequalities.
Contrasting the testimony and actions of those called to the witness stand provides all that is necessary to discriminate between personages of ethical character and those without any morals or ethics. When Bob Ewell perjures himself on the stand, testifying that Tom has struck his daughter and raped her--"I seen that blackn--- yonder ruttin' on my Mayella"--his evil intentions of impugning Tom's character and "saving face" for his daughter and himself are apparent:
"Mr. Tate testified that her right eye was blackened, that she was beaten around the--"
"Oh yeah," said the witness. "I hold with everything Tate said." [that Tom allegedly struck Mayella]
On the other hand when the ingenuous and candid Tom Robinson testifies, he tells the truth: "Did you ever go on the place [the Ewells] again?"
"Yes suh....Well, I went losts of times"....
"Why did you go inside the fence a lot of times?"
..."She'd call me in, suh. Seemed like every time Ipassed by yonder she'd have some little somethin' for me to do--choppin' kind=lin', totin' water for her....I was glad to do it, Mr. Ewell didn't seem to help her none, and neither did the chillun, and I knowed she didn't have no nickels to spare."
A man of great integrity, Tom even tells the truth when doing so is a social gaff. For, in his innocence and honesty he tells Atticus Finch that he helped Mayella because he "felt sorry" for her, an admission that no self-protecting black of the 1930s in the South would make. As a result of Tom's overstepping his social position, Maycomb's jury falsely condemns him because he has violated the mores of a racially segregated society.