The identity of the culprit is Dr. Grimesby Roylott. He is strongly suspected from the beginning because, for one thing, he had an obvious motive for killing Julia Stoner two years ago and he has the same motive for trying to kill Helen Stoner now. Under the terms of his deceased wife's will, Roylott would be forced to turn over one-third of his dwindling capital to either stepdaughter if she got married. So he wants to kill the girls when they become engaged, and, being a doctor and a scientist, he knows how to do it without getting caught. Helen's life has only become endangered recently because, as she tells Sherlock Holmes at their initial meeting:
"A month ago, however, a dear friend, whom I have known for many years, has done me the honour to ask my hand in marriage. His name is Armitage—Percy Armitage—the second son of Mr. Armitage, of Crane Water, near Reading."
Roylott might be able to intimidate his stepdaughters and refuse to give them any part of their mother's bequest. However, he knows that if one of the girls got married he would have to deal with her husband. In those Victorian times most women knew little about practical matters and their husbands took charge of such things. Roylott is iron-bound by law to part with his capital if either girl marries.
Helen describes her stepfather as a powerful and violent man. And he proves this when he storms into Holmes' and Watson's rooms to demand to know what Helen has been telling them. He says:
"I am a dangerous man to fall foul of! See here.” He stepped swiftly forward, seized the poker, and bent it into a curve with his huge brown hands."
Although Helen does not suspect her stepfather of trying to murder her, Holmes feels sure that Roylott was somehow connected with Julia Stoner's death and may be connected with a plot to murder Helen. The presence of a band of gypsies on the grounds at Stoke Moran misleads Holmes, and is intended by the author to mislead the reader. Holmes thinks that Helen's dying reference to a "speckled band" may refer to a band of gypsies wearing speckled head bands, or something of the sort. Dr. Roylott does not reappear in the story until after he found dead in his room with the "speckled band" wrapped around his head; but his threat of violence hangs like a cloud over the rest of the story. The reader half-expects Dr. Roylott to return home from London unexpectedly and perhaps trying to kill Holmes and Watson with one of his hunting guns. Watson has brought his revolver; there could be a shootout. Helen has told the detective that her stepfather planned to stay in London all that day; but it seems possible that he might become suspicious of both Helen and Holmes and return to Stoke Moran much earlier than anticipated.
Since Dr. Roylott is killed by his own poisonous snake, there is no need to bring in the police. Helen wants the matter hushed up anyway. The doctor's death is attributed to his being bitten while handling his snake, which was one of the many exotic animals he studied. Helen will inherit the house and all the money, and she will be able to go ahead with her wedding to Percy Armitage.