Harper Lee believes that racism is a "sin." Sin is often defined as a "willful violation" of either divine of human law. Tom Robinson's unjust conviction is an example sins on both fronts.
Another sin along these same lines is prejudice in general. The ostracizing of Boo Radley is sinful in that it disrespects a kind man's dignity. Sin, once discovered, is usually accompanied by shame. Jem, Scout and Dill definitely see their previous actions toward this gentle soul as sinful.
Still another sin is lesser, but if one takes into account the unjust nature of sin, then the modern education system is sinful. Scout has had a better education in life than any of her peers, yet she is put down by the institutionally trained Miss Caroline. Lee is a believer in all kinds of knowledge; the fact that Scout is put down for her brand of learning is indeed sinful in the author's eyes.