Early in To Kill a Mockingbird, the novel paraphrases Franklin D. Roosevelt's inaugural address: "the only thing we have to fear is fear itself." Right on: fear can be very scary when it hijacks people's reason and compassion for others. As another great statesman, Yoda, put it, "Fear leads to anger. Anger leads to hate. Hate leads to suffering." And where does that all leave us? The Dark Side, or in this case, racism, injustice, harassment, and senseless killing. Fear makes people lash out against what scares them to restore the familiar order, even if they have to destroy innocent lives along the way.
Maycomb is afraid that if Mayella's accusation is revealed as false, other African-American men will commit the crime of which Tom is falsely accused.
For Scout, growing up means losing her fear of the unknown.