Dr. Roylott is a wicked and selfish man, and he is half-insane; but he is obviously intelligent. He spends much of his time in scientific research connected with exotic animals. He may be one of the most knowledgeable men in this field in all of England. He was able to perpetrate the murder of Julia Stoner without getting accused of the crime. He demonstrates his practical intelligence by tracing his stepdaughter Helen Stoner to 221B Baker Street within a very short time after she arrives there herself. When he discovered that she was not at Stoke Moran, he immediately deduced that she had gone to London. He went to Leatherhead and asked questions. He soon learned that a young woman dressed in mourning and wearing a veil had taken a train to London early that morning. Helen was wearing a dark dress and a heavy veil in the hope of disguising herself, should her stepfather try to follow her. But she was only making herself more conspicuous and easier to follow. There would be very few women traveling by train alone in those Victorian times. When Roylott got to London, he questioned cabbies and quickly found the one who had taken his stepdaughter to Baker Street. He probably did not know that Sherlock Holmes lived at 221B, but he could have made some inquiries in the neighborhood before barging into Holmes' and Watson's flat. He did know about Sherlock Holmes because the detective was already becoming famous for his intellectual powers. Roylott tells him:
"You are Holmes, the meddler.”
“Holmes, the busybody!”
“Holmes, the Scotland Yard Jack-in-off."
Dr. Roylott is sufficiently intelligent to realize that he ought to lie low for a while, but he may think that he has scared Holmes off with his threats and his demonstration of his mighty physical power in bending a steel poker into a ninety-degree angle. In any case, he is under time pressure. Helen is due to be married soon, and he cannot afford to give her one-third of his capital, as required under the terms of his deceased wife's will. So he tries sending his poisonous snake into Helen's room that very night, not suspecting that Holmes and Watson are hiding there in the dark.
Although Roylott is unquestionably intelligent, his one "flaw" is his violent temper. In India he beat a man to death and served a long term in prison. At Stoke Moran he has made many enemies with his temper-tantrums. Helen tells Holmes and Watson:
“Last week he hurled the local blacksmith over a parapet into a stream, and it was only by paying over all the money which I could gather together that I was able to avert another public exposure."
The author of "The Adventure of the Speckled Band," Arthur Conan Doyle, must have decided to make his villain violent and brutal, rather than sinister and furtive for example, in order to create the feeling of impending danger which prevails throughout the story. The reader feels apprehensive while Holmes and Watson are examining the exterior and interior of Stoke Moran. Dr. Roylott could return from London at any moment. If he caught Holmes and Watson on his property, he could feel justified in shooting them, on the premise that "A man's home is his castle." No doubt Dr. Roylott has access to a number of rifles and shotguns and enjoys shooting birds and rabbits.