Society is culpable for the majority of what happened at the end of book one. Throughout the entire first book, it is obvious that Bigger constantly feels uncomfortable around white people. If Bigger was a white man himself, this would have never happened and he wouldn't have so much fear in every move he makes. The reason that he killed Mary was because he was scared he was going to lose his job and give off the wrong impression. If Bigger was white and didn't have that pressure from society, he would have been able to deal with the situation in a better way. I do believe that when Bigger proceeds to destroy Mary's body after she died is completely Bigger's fault. His actions were severe and unnatural and Bigger had no reason to dispose her body in those extreme ways.
-Linnea