Mr And Mrs Simpson

Riddle: Mr. and Mrs. Simpson are very rich and they live in a mansion. They are so rich that they have a maid, a butler, a mailman, a painter, a builder, and a fat cat named Midnight. One day Mr. and Mrs. Simpson go to see a movie. When they returned the maid is dead on the floor and covered in claw marks. The butler said he was feeding the cat, the mailman said he was going through the Simpson's mail, the painter said she was painting a picture and the builder said he was pulling old nails out of some boards. And the cat said 'burp'. The Simpson's immediately knew who did it. Who was the killer?