What happened to Dr. Roylott in "The Adventure of the Speckled Band"?
Holmes and Watson spend the night in Helen Stoner's bedroom, waiting to see what will happen. At three o'clock in the morning by the nearby church bell, they hear the low whistle which Helen described to them at Baker Street and which she tells them her sister Julia had described to her two years earlier.
The instant that we heard it, Holmes sprang from the bed, struck a match, and lashed furiously with his cane at the bell-pull.
Holmes sees the poisonous snake on the bell-pull. The whistle was being used to summon it back from Helen's bed and through the ventilator into Dr. Roylott's adjacent bedroom. They hear a "horrible cry" from the room next door. They go into Roylott's room and find that he has been bitten by his own snake.
His chin was cocked upward and his eyes were fixed in a dreadful, rigid stare at the corner of the ceiling.
The snake has coiled itself around the dead man's head. It's markings show clearly that it is the "speckled band" Helen's sister Julia told her about as she was dying. Holmes expresses the moral of the story when he tells his friend Watson:
Violence does, in truth, recoil upon the violent, and the schemer falls into the pit which he digs for another.