Racism is corrosive for a society because it teaches people to make judgments about others on the basis of the way they look or assumptions that they might make about people from different cultures. Racism allows people to justify all sorts of indignities and horrors to be visited on people from other cultures by saying that the other people are inferior or somehow less than human in some way.
One of the most powerful statements of racism appeared in the original U.S. Constitution. Because some states did not permit slavery while others did, a debate arose as to the role that slaves should play in terms of counting population. The slave-owning states did not want to have their slaves counted as full citizens, because of the resulting tax burden, but they also did not want them left out of the population count, as states received representation in the House of Representatives based on relative population. The "three-fifths" compromise allowed slaves to be counted as 0.6, or 3/5, of a person for both purposes.
The tragedy of "To Kill a Mockingbird," and thousands of true-life stories just like it, could only come in a racist society. The black man in the story clearly could not have committed the crimes of which he was accused, but the all-white jury simply would not acquit him, instead sending him to prison, where he would be shot "trying to escape." These types of stories are part of the hateful legacy of racism.