In "The Adventure of the Speckled Band," what does the evidence Holmes has discovered suggest?

At the end of the story, after Helen Stoner has been saved from death and Dr. Grimesby Roylott has been killed by his own snake, Holmes tells Watson all about his deductions from the evidence found at Stoke Moran.

My attention was speedily drawn, as I have already remarked to you, to this ventilator, and to the bell-rope which hung down to the bed. The discovery that this was a dummy, and that the bed was clamped to the floor, instantly gave rise to the suspicion that the rope was there as a bridge for something passing through the hole and coming to the bed. The idea of a snake instantly occurred to me, and when I coupled it with my knowledge that the doctor was furnished with a supply of creatures from India, I felt that I was probably on the right track.

So Holmes was expecting a snake when he and Watson were sitting in silence in the former bedroom of Julia Stoner which had recently been assigned to her sister Helen. What the two men do not realize is that the snake is actually there on the bed for several hours. Holmes doesn't see it until, at around three-thirty in the morning, he hears the low whistle his client Helen had told him about. That means Dr. Roylott is summoning it back up the bell-pull, through the ventilator, and into his adjoining room. Holmes strikes a match and lashes at the speckled band climbing back to the ventilator. Since Holmes had been sitting on the side of the bed, he must have been sitting very close to the coiled snake for several hours without knowing it.

Holmes sees other evidence when he examines the bedrooms earlier in the day. Dr. Roylott has a dog-leash with a noose fashioned at the end of it. He has a saucer containing some milk on top of a steel safe. Holmes deduces that Roylott has trained a snake to return through the ventilator at the sound of a low whistle by rewarding it with the milk and then locking it in the safe. Holmes examines the plain wooden chair in Roylott's room and sees evidence that he must have been standing on it in order to put his poisonous snake through the ventilator. No doubt he would also have been standing on that chair to recapture the snake when it returned, but when Holmes lashed it with his cane the snake returned before the doctor was prepared to slip the noose around it. That was how he came to be fatally bitten. When Holmes and Watson see him:

Beside this table, on the wooden chair, sat Dr. Grimesby Roylott clad in a long grey dressing-gown, his bare ankles protruding beneath, and his feet thrust into red heelless Turkish slippers. Across his lap lay the short stock with the long lash which we had noticed during the day. His chin was cocked upward and his eyes were fixed in a dreadful, rigid stare at the corner of the ceiling. Round his brow he had a peculiar yellow band, with brownish speckles, which seemed to be bound tightly round his head. 

There is also evidence that Helen was moved into Julia's former bedroom in order to be in the bed under the ventilator and beside the dummy bell-pull.

"By the way, there does not seem to be any very pressing need for repairs at that end wall.”

“There were none. I believe that it was an excuse to move me from my room.”

Arthur Conan Doyle titled his story "The Adventure of the Speckled Band" and never mentions the word "snake" until the very end. He did not want his readers to get the idea of a snake being used for murder because that would spoil the ending. Helen only uses the words "speckled band" when telling about Julia'a death, and even when Holmes and Watson see it wrapped around Dr. Roylott's head, 

“The band! the speckled band!” whispered Holmes.