What forced Helen's stepfather to leave?
The formidable Dr. Grimesby Roylott was not exactly forced to leave the rooms on Baker Street where he had burst in on Sherlock Holmes and Dr. Watson shortly after Helen Stoner left. He thought he could intimidate Holmes and get him to tell what he had learned from his stepdaughter. First off, he asks:
"What has she been saying to you?”
But Holmes would never reveal a client's confidential information, and least of all to someone like Dr. Roylott, who is accustomed to walking all over people. Holmes totally ignores Roylott's questions and talks about the weather. Then finally he asks the angry stepfather politely to close the door when he leaves. Roylott understands that he is not going to get the information he has come there for, so he issues a threat with a demonstration of his remarkable strength.
“I will go when I have said my say. 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.
“See that you keep yourself out of my grip,” he snarled, and hurling the twisted poker into the fireplace he strode out of the room.
He makes it apparent that he is indeed "a dangerous man to fall foul of." This incident casts an ominous cloud of danger over the rest of the story, because Holmes and Watson are carefully examining the interior and exterior of Roylott's big manor house called Stoke Moran, and there is always the possibility that the owner will return unexpectedly and catch them trespassing. Holmes has asked Watson to bring his pistol. The reader might well expect a confrontation in which Roylott attacked both men, perhaps with a shotgun, and Watson was forced to shoot and kill him. This could look very bad for Watson, since he and Holmes were the intruders and "A man's home is his castle."
What forced Helen's stepfather to leave Baker Street was Holmes' cold refusal to give him any information about Helen's visit. Fortunately for Helen, the doctor does not try to grill her about her visit to see Sherlock Holmes when he gets home. He probably does not want to frighten her at that time because he fully intends to murder her with his poisonous snake very soon. He apparently does not think that Holmes will come to Stoke Moran. He probably thinks he has frightened the detective into staying out of his affairs. And furthermore, Helen doesn't have any money to pay for the services of a private detective. Her visit to Holmes will only make Roylott feel that he must dispose of her quickly, since she is obviously becoming suspicious. He has no choice but to continue to try to kill her, since he cannot possibly pay her the money he will owe her when she gets married. He got away with murdering her sister Julia two years before, and he thinks he can get away with doing the same thing to Helen. Probably the one flaw in Roylott's thinking is that he doesn't realize that Holmes will help people without payment if their case intrigues him or if he feels sympathetic towards them. In this case, Holmes is both intrigued by the mystery and sympathetic towards a young woman in distress.