Why did Atticus choose to defend Tom even though no one else wanted him to?

In Chapter 9, Scout asks her father why he is defending Tom Robinson when the community believes that he shouldn't be defending a Negro. Atticus tells Scout that the main reason he decided to defend Tom was because he wouldn't be able to hold up his head in town, represent Maycomb in the legislature, or tell his children what to do anymore. In Chapter 11, Scout insists that Atticus must be wrong, and he explains to her that Tom's case "goes to the essence of a man's conscience" (Lee 66). Atticus says that he couldn't live with himself if he didn't defend Tom to the best of his abilities. He says,

"The one thing that doesn't abide by majority rule is a person's conscience" (Lee 66).

Atticus is a morally upright individual who is forced to follow his conscience instead of popular opinion. He would not be able to live with the guilt of not defending Tom Robinson. Despite the opinions of others, Atticus follows his heart and does the right thing by defending Tom.