Dr. Roylett's motive was money. His wife's will stated that the total income they would get from her estate would be 100 pounds. Due to the fact that agriculture prices had fallen, the estate was now worth about 750 pounds. Upon marriage, according to the will, each girl would get 250 pounds. That decrease in his worth would "cripple him to a serious extent".
The reason Roylett's plan backfired was because of the genius of Sherlock Holmes. Holmes found the bell pull that was connected to the ventilator. The ventilator had no purpose, opening only to the bedroom next store instead of to the outside air. He noticed that the bed had been fastened to the ground so that it could not be moved. He figured something had to crawl down that rope. When the snake crawled down the rope, Holmes attacked it with its cane, sending it back to through the ventilator. It was so angry that it attacked the first person it saw, which was Dr. Roylett. The snake he had chosen was the deadliest snake in India and killed within ten minutes of being the victim being bitten.