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.