Dr. Roylott slipped his poisonous snake through the ventilator. It slithered down the dummy bell-rope onto the pillow beside the sleeping girl’s head. Why didn’t it try to escape from its cruel captivity now that it was at least partially free? The snake comes from a tropical climate and the weather is remarkably cold, as the author suggests many times.

"I am glad to see that Mrs. Hudson has had the good sense to light the fire. Pray draw up to it, and I shall order you a cup of hot coffee, for I observe that you are shivering.”

“It is a little cold for the time of the year,” said Holmes.

The central portion was in little better repair, but the right-hand block was comparatively modern, and the blinds in the windows, with the blue smoke curling up from the chimneys, showed that this was where the family resided.

The tropical reptile's natural inclination would be to seek warmth. Dr. Roylott must have known that. On each of the nights the snake crawled down the bell-rope it would not only have remained on the bed, but it would have crawled under the bed-covers and curled up right beside the sleeping Julia. The snake had no reason to attack the girl whose body heat was keeping it warm, but sooner or later, as the murderer knew, the girl would turn over in bed and right on top of the swamp adder. When that happened, the snake would bite her through her nightgown. She would scream, and Roylott would immediately blow his whistle to summon the “speckled band” back up the bell-rope and through the ventilator, where he could capture it and return it to the steel safe.

The reader—and especially the female reader—can imagine how horrible it would be to have a snake as a bed companion. The same thing would have happened to Helen if her sister hadn’t told her about that mysterious whistle. Helen heard it on only one occasion and went to see Sherlock Holmes immediately next morning—but she must have had the swamp adder in bed with her for at least one night.

Dr. Roylott was guilty of murdering Julia by sending a highly poisonous snake into her bedroom, which was adjacent to his. The snake would crawl through the ventilator and down the dummy bell-rope directly onto the bed beside the sleeping girl's face. The snake had no intention of killing Julia. It had been trained to climb up and down the bell-rope and through the ventilator. When it got into Julia's bedroom in the dead of night it was attracted by the warmth generated by the girl's body. Doyle does not have Watson surmise that the snake actually crawled under the blankets to be closer to the warm body, but that must have been what occurred and what must have been Roylott's intention. The snake was a cold-blooded reptile from the hot climate of India. It would not be likely to try to escape from the house into the outdoors, especially since Julia was killed in the wintertime after Christmas. It would be extremely cold outdoors and the bedroom itself would probably be cold as well.

Helen comes to see Holmes at Baker Street early in April. The weather might be somewhat warmer, but it is mentioned several times in the story that it is still very cold. For instance, when Dr. Roylott makes his appearance in Holmes' sitting room and threatens violence, the unflappable detective says:

"It is a little cold for the time of the year."

It is horrible to think of how Julia was killed. The snake would actually crawl under the covers with her night after night seeking warmth. Roylott knew that sooner or later his stepdaughter would turn over in her sleep and be bitten through her nightgown when she was right on top of the snake. Roylott used his whistle to summon the snake back up the bell-rope and through the ventilator into his own room when he heard "the wild scream of a terrified woman." The stricken Julia got out of bed and met her sister in the corridor, so there was no chance of Helen seeing the snake gliding up the bell-rope or disappearing into the ventilator.

Dr. Roylott wanted to kill Julia because she had become engaged to be married. He was legally obligated to give her one-third of the income from her mother's estate. He couldn't afford to part with that much money because he was heavily in debt. Helen was safe enough for two years after her sister's death, but then she became engaged and Roylott felt compelled to murder her in the same way he had murdered her sister Julia. But the big difference was that Helen had seen how Julia had somehow been killed in a locked bedroom under mysterious circumstances, and Helen had been told about the strange whistle by her dying sister. It was the whistle more than anything that frightened her into traveling directly to Baker Street to ask Sherlock Holmes for advice.

Julia’s life had been lonely and difficult, and was just starting to get better right before her death.

Sherlock Holmes’s newest client, Helen Stoner, explains that she and her sister did not lead lives of pleasure before she died.

No servant would stay with us, and for a long time we did all the work of the house. She was but thirty at the time of her death, and yet her hair had already begun to whiten, even as mine has.

Her life was beginning to improve, because she was engaged to “a half-pay major of marines” before she died.

One of the causes of the sisters’ grief is their stepfather, Dr. Roylott.  He did not say he was opposed to the marriage, but it was soon after that Julia died.  This made her sister Helen suspicious of him, and even consider him a suspect.

Although we usually are sympathetic toward the victim, the story probably causes the reader and Dr. Watson to be even more sympathetic.  Holmes is rarely moved, but he was intrigued by the case and accepted it.  He was able to prove that Dr. Roylott did murder Julia.