Not surprisingly, most of the quotes about justice and injustice come from Atticus Finch in Harper Lee's To Kill a Mockingbird, though of course these are common themes in the novel and can show up in many characters' conversations and situations.
Atticus talks about race and reminds us that the person who treats another human being in an unjust manner is of the lowest class.
"As you grow older, you’ll see white men cheat black men every day of your life, but let me tell you something and don’t you forget it—whenever a white man does that to a black man, no matter who he is, how rich he is, or how fine a family he comes from, that white man is trash."
During Tom Robinson's trial, Dill is sickened at the way in which Mr. Gilmer is questioning Tom.
"The way that man called him 'boy' all the time an' sneered at him, an' looked around at the jury every time he answered-[…] It ain't right, somehow it ain't right to do 'em that way. Hasn't anybody got any business talkin' like that—it just makes me sick."
Dill is a child, yet even he is aware, though he can hardly articulate it, that it is an injustice to treat another human being as if he is inferior just because of his skin color.
The most horrible injustice in the novel, of course, is the fact that Tom Robinson is on trial for raping a white girl when in fact all he did was show a young girl some basic human kindness. That is an obvious injustice; however, a more subtle injustice is Mayella Ewell being forced into a false accusation by both her father and society. Atticus describes it this way:
"She has committed no crime, she has merely broken a rigid and time-honored code of our society, a code so severe that whoever breaks it is hounded from our midst as unfit to live with. She is the victim of cruel poverty and ignorance, but I cannot pity her: she is white. She knew full well the enormity of her offense, but because her desires were stronger than the code she was breaking, she persisted in breaking it."
It is an unjust society (and family, in this case) which does not allow two people--of whatever color--to be friends. It is an unjust society which harbors such extreme poverty, and it is an unjust family situation which requires Mayella to lie about an innocent man who took pity on her--something her own father never did. It must be added that Mayella also made an unjust choice.
Atticus tells Jem that injustice exists everywhere, and it always will:
"There's a lot of ugly things in this world, son. I wish I could keep 'em all away from you. That's never possible."
Despite that, Atticus fights for the life of an innocent black man at a time and place where he is likely to lose. His closing argument is the best explanation of justice in the novel. It reads in part:
Now, gentlemen, in this country our courts are the great levelers. In our courts, all men are created equal. I’m no idealist to believe firmly in the integrity of our courts and of our jury system. That’s no ideal to me. That is a living, working reality.
To Kill a Mockingbird is a novel which explores the issues of justice and, conversely, injustice. It does so in terms of race, class, and character. No one category of people is seen as better than others, and injustice can be found everywhere. In general, though, for every injustice, there is someone fighting for or representing justice, which is undoubtedly one of the reasons the novel is considered a classic.