The rope, the ventilator and Julia's clamped bed all represent Dr Roylott's devilish plan to murder his own daughters for the sake of their money. He uses one of his exotic pets, a snake, to accomplish this, and succeeds in the case of Julia, if not her sister Helen. On successive nights he drives the snake, which he has trained, through the ventilator from his own room and into Julia's, and down the rope onto her bed. One time she hears the whistle which he uses to summon the snake back to him, and puzzles at it. Finally, one night, when the snake slithers down on her bed, it also stings her, killing her. When dying in the arms of her sister, she can provide no clue as to what happened, except to refer vaguely to a speckled band, which initally misleads Holmes into thinking that maybe she meant the headgear worn by the gypsies with whom her father is friendly. However, this description actually refers to the appearance of the snake.
Soon enough, of course, Holmes gets on the right track and in his usual dramatic way, finally reveals the snake, in the act of killing its own master, the fiendish Dr Roylott. Having engaged Watson to accompany him on a night vigil in Julia's old room, Holmes is ready for the snake when it comes hissing through the ventilator, strikes it with his cane and therefore succeeds in driving it back through the ventilator to Roylott's room, where it turns on him. Holmes observes:
Violence does, in truth, recoil upon the violent, and the schemer falls into the pit which he digs for another.
In other words, the doctor has got his just deserts. The creature which he used to kill one of his daughters, and also planned to kill the other, ends up killing him.