What point is Doyle trying to make in "The Adventure of the Speckled Band"?

The author is not mainly concerned with making a point or proving a thesis. The story is an adventure yarn, the type of story that Graham Greene would call an "entertainment." The main mystery Sherlock Holmes is called upon to solve is: How could Helen Stoner's sister Julia be killed in her bedroom when the shutters and the door into the corridor were securely locked? This is an example of a subgenre called a "Locked...

The author is not mainly concerned with making a point or proving a thesis. The story is an adventure yarn, the type of story that Graham Greene would call an "entertainment." The main mystery Sherlock Holmes is called upon to solve is: How could Helen Stoner's sister Julia be killed in her bedroom when the shutters and the door into the corridor were securely locked? This is an example of a subgenre called a "Locked room murder mystery." The prototype is Edgar Allan Poe's story "The Murders in the Rue Morgue," in which two reclusive women were murdered in a room in which the door and all the windows appeared to be securely locked. When Holmes deduces the answer to his own locked-room murder mystery from what he observes in Dr. Roylott's room and the directly adjacent room which Helen is now occupying, he solves the mystery of Julia's agonizing death two years ago and also forestalls Roylott's plan to murder Helen. Doyle gives a "point" or moral to the story by having Holmes quote from the Old Testament.

Violence does, in truth, recoil upon the violent, and the schemer falls into the pit which he digs for another. 

Dr. Roylott is certainly a violent man. He makes a great show of potential violence when he bursts into Holmes' room at Baker Street and tells him:

Don't you dare to meddle with my affairs. I know that Miss Stoner has been here. I traced her! 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.

This is the only time Dr. Roylott will appear in the story while he is still alive, but it is sufficient to establish that the essence of the tale is a conflict between Roylott and Holmes, the man of violence and the man of intellect.