Think about the majority of people in To Kill a Mockingbird, they hate Tom Robinson simply because he's black and they believe he did the horrible crime without any real evidence. The majority rules that Tom was guilty, even though they are wrong. It's reasonable to believe that at least one or two of the jurors knew in the back of their mind that Tom was innocent, but the jurors were simply unable or unwilling to stand up to the majority rule. So one person's conscience, Atticus Finch's conscience, did not follow the majority rule. All it takes is one person to stand up to the majority and all of a sudden it doesn't seem so impossible to take a stand. Atticus' conscience wouldn't let him just let Tom go down without a fight.