In this day and age, it would be nice to think that everybody knows the simple fact: "racism is bad". To Kill a Mockingbird goes beyond that simple message. Not all forms of racism are the same, some are derived from hate, some from fear, some from the thought of superiority, some from all these together.
Throughout the novel, it shows some who have the inability to see that even someone who looks, and talks, and acts very different from oneself is actually the same as every other human being.
To Kill a Mockingbird presents a flawed justice system. In our current society, the jury determines guilt or innocence based on the facts proven during the trial. But in the novel, a group of biased white men give a verdict they had decided on before they even entered the courtroom.
Morality & Ethics
To Kill a Mockingbird examines the difference in morals between an individual and the community when each has a different standard of right and wrong. Atticus Finch has strong morals in racial justice, while most others in Maycomb discriminate against blacks.
Some of To Kill a Mockingbird's main characters are children, so youth is an obvious theme in the text. The story shows that children can sometimes have a different way of looking at things than adults.
Symbolism: Rabid Dog
Here is a link to a great explanation of the symbolism of Tim Johnson the rabid dog