Marya Timofeyevna, had been ‘stabbed all over’ with a knife, and was lying on the floor in the doorway, so that she probably had been awake and had struggled and fought back with her killer.
Definitely Pyotr's doing because he offered to get rid of Marya for Nikolay and bring him Liza to 'make up'. Now Marya is dead and Liza and Nikolay are together somewhere.
