Mary died because Bigger smothered her with a pillow until she could no longer breath. Mrs Dalton asked Bigger if he knows what happened to Mary and he lies and says "I---I was asleep..." this shows that he does not care about his actions and isn't afraid to lying or getting into touble. Bigger has changed because he killed someone and lies to the Dalton's about it.
Bessie and Bigger are discussing a plan to black mail the Dalton's. They are going to pretend that they are the kidnappers and ask for money and they will give back Mary.