I read to kill a mocking bird twice and covered it in school. So I have a good idea but I’m not completely sure and at least you’ll have something.
But Atticus defends Tom Robinson because he is a man, he is a human like any other. He takes this case seriously because he wants to teach his children the rights and wrongs of life. The innocent and the guilty. Atticus knows very well that Tom is innocent, but he knows that with Tom Robinson being a black man in this time he will be guilty no matter how good the evidence proves him innocent.
I hope this helps you! ;))