During Helen Stoner's visit to Sherlock Holmes office, he noticed several aspects of her personality that indicated that she was terrified and suffering from mental anxiety as well as physical exhaustion. He also uncovered bruises on her wrists which was an indication that she was being physically abused by her stepfather.
After Helen Stoner's visit to Sherlock Holmes office, her stepfather, Dr. Roylott storms in demanding to see Holmes. He is aggressive and threatens Holmes with violence, making threats.
"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." (Doyle)
Dr. Roylott is a violent man who has a motive for murder, money. Once Holmes uncovers the will of his late wife and discovers the conditions that award the two daughters, Julia who was murdered two years earlier, and Helen, upon their marriage, he becomes convinced that Dr. Roylott has systematically eliminated one daughter and is presently setting up Helen for the same fate.
He has an idea of the method of murder, but needs to prove it by spending the night in the darkened bedroom of Helen Stoner. He does so with Watson by his side and then discovers the last piece of the puzzle, Dr. Roylott's murder weapon, the snake that he brought back from India.