1/12 Lamb to the Slaughter

In short, the lady kills her husband with a leg of lamb because he was planning on leaving her. She then ransacks the room to make the scene appear to be a robbery, then goes out to the store to fabricate her alibi. When she returns home she calls the police and they arrive at the crime scene to question her and such. After a long night of gathering evidence and whatnot the cops are about to retire but the kind lady insists on preparing them a meal, which happened to be the leg of lamb. Unknowingly the unsuspecting police gratefully gobble up the lamb, destroying the murder weapon, and ensuring the case will go cold.