Above all else, Atticus is a realist. He is not angry with Jack for spanking Scout, because he knows "she earned it." He understands that the case he has accepted--defending a black man who has been accused of raping a white woman--is a losing proposition. He realizes that he and his family will make enemies by defending Tom Robinson, and he knows that he has no chance of winning.
"The jury couldn't possibly be expected to take Tom Robinson's word against the Ewells'."
But Atticus' conscience guides him in making the decision, primarily because he could not "face my children otherwise." Atticus allows Scout to deliberately overhear them talking so that she knows the facts of the case; so that she knows how much he cares for and respects his children; and so that she can wonder what mysterious power Atticus employs to know that she was eavesdropping.