This is a good essay topic.  For any essay, you need to begin with a theme that will help focus your thesis.  For example, how about the theme that justice is defined by the individual, rather than society?  This would be an interesting theme for your essay, because you could explore reasons why Atticus is defending Tom Robinson and why Boo Radley killing Bob Ewell was just.

Atticus says that he took Tom Robinson’s case because he couldn’t live with himself if he didn’t.  In chapter 11, Atticus tells Scout why he had to take the case:

This case, Tom Robinson’s case, is something that goes to the essence of a man’s conscience—Scout, I couldn’t go to church and worship God if I didn’t try to help that man.

In this case, Atticus defines justice different than society.  Most people in the town disapprove of his defending Tom, because he is black.  Atticus does it because he feels that Atticus is being unfairly targeted due to prejudice.  For Atticus, the society’s definition of justice is wrong, and he is willing to face public ridicule and even physical danger to do what he feels is right.

Another instance of justice being defined by the individual is when Boo Radley kills Bob Ewell.  He kills Ewell to defend Scout.  Although in most cases it is wrong to kill a man, Boo does it anyway because he wants to defend the innocent children targeted by Ewell because of his embarrassment at trial by their father.  Heck Tate chooses not to pursue the matter, because being in the limelight would be terrible for Arthur Radley, who is terribly shy.  Heck decides to tell everyone that Bob Ewell fell on his knife.

 I never heard tell that it’s against the law for a citizen to do his utmost to prevent a crime from being committed, which is exactly what he did. (Chapter 30)

 In this case, justice would not be done by arresting or even questioning Boo Radley, or having a trial.  Justice is done by leaving him alone.