I don't think that society had a very big, if any impact on mary's murder. As we've seen before, Bigger is just a violent person who acts without thinking. He's paranoid and freaks out when put under any sort of stress. He knew that Mrs.Dalton was blind and he could have easily slipped by her. Instead, he saw that mary was making a little noise and blew it out of proportions and put a pillow over her face. Since he was under stress, he was not thinking, which is the basis to most of his problems. And as to the events leading up to mary's murder, anyone of any race could have decided not to take her to the university and instead to go out and get drunk, he did that because he worked for her and she told him to. I think the only ways society way culpable was that they frown upon murder, thus causing Bigger to get rid of the body