Atticus Finch is a lawyer, but he also strongly believes in the justice system. He takes Tom Robinson’s case even though he knows that it will be difficult and he probably won’t win.
Scout is confused because Atticus is just doing his job, and people don’t like the fact that he wants to do it well.
The court appointed Atticus to defend him. Atticus aimed to defend him. That's what they didn't like about it. It was confusing. (ch 16)
Whereas some people might have decided to not really try, Atticus really does want to present the best case for Tom Robinson. This is because he believes in justice. When he explains this to the jury, he is trying to get them to understand their responsibility.
I'm no idealist to believe firmly in the integrity of our courts and in the jury system- that is no ideal to me, it is a living, working reality. (ch 20)
Atticus is an important member of the Maycomb community. He is not just a defense attorney. He is also a legislator. He is well-respected and has a serious job to do, and he takes that responsibility seriously.
Another reason Atticus approaches the court’s justice so gravely is that he wants to instill values in his children. He wants them to learn right from wrong, and he wants to model that behavior.
Atticus is well able to see the flaws in the American justice system but continues to believe in it as the mark of a civilised country. It is also the system that he is part of and he wants to do all he can to improve it. He is naturally very discouraged after Tom Robinson's trial but still he does not give up hope. Essentially this is because he believes that overall, human beings are more good than bad, and that things can progress. As he tells Scout at the end, 'Most people are (nice), Scout, when you finally see them.' The book thus ends on a positive note regarding human nature.