Ideal: a jury of one's peers dispassionately determine guilt or innocence based on the fact. Reality: a group of white men who aren't influential enough to get out of jury duty have decided the case before they even enter the courtroom. In To Kill a Mockingbird, justice is a privilege, not a right. You want a fair trial? Well, we sure hope you were lucky enough to be born white.
In To Kill a Mockingbird, the criminal court system may be broken, but it's still the best chance for justice.
To Kill a Mockingbird contrasts court-justice and vigilante-justice to show that they both have strengths and weaknesses.