why does atticus think he has to defend tom robinson?

Atticus believes that everyone is equal before the law: therefore just because Tom Robinson is black, doesn't mean that he should not defend him. He says to Scout many times, if he did not defend this man, he could never tell Jem or her (Scout) not to do something gain. Atticus has to do the right thing even if this means sacrificing his life and reputation in the town.