I think mary had a good heart and probably had a truer nature to help the oppressed than her father. She was just niave and was just beginning to learn about how she might funnel her desires to equalize black and white, poor and rich and may have been enamored by Jans party platform. Unfortunatley she did rather look at bigger and the neighborhood and restaraunt as one might look at a bug under a microscope because no matter what she aspired to be, unpredjiduced, she still felt the underlying difference between them.
as far as biggers fears after killing Mary..I think he very much wants to live and would do anything to cover up his crime, again with no thought to how that will affect Jan an innocent man. He may think 'so what? he's white, it's just paybacks for all the injustices done to me and mine.'