In "The Adventure of the Speckled Band," Dr. Roylott dies from being bitten by a poisonous snake.

Dr. Roylott has devised an ingenious and diabolical scheme to murder both his stepdaughters before they can marry and gain part of the estate he has inherited from his dead wife. In the case of Miss Helen Stoner's sister, he is successful, which is why Helen hires Sherlock Holmes.

Roylott's scheme consists of insisting the daughter sleep in the room next to him. He has a ventilation shaft put between the two rooms. He sets the snake to slither through the ventilator and down a pull cord that dangles by the daughter's bed. The cord is meant to be connected to a bell that can call a servant, but it has never been connected. At the bottom of the cord, the snake will drop to the daughter's bed and bite her before slipping away again. It is "the speckled band" Helen's sister exclaims about seeing before she dies.

As Holmes and Watson wait at night in Helen's room, they see the snake coming down the bell cord and realize what is going on. Holmes frightens the snake so that it hurries back to Roylott's room. Once there, the snake bites and kills Roylott.

This is what is called "poetic justice," which is when a person gets a just reward or punishment for their actions. Roylott, being a murderer, deserved to die at the hands (or bite) of his own murder weapon.