Conan Doyle uses several clues which identify Dr Roylott as the perpetrator of the crime. One of the first pieces of information revealed about him by his stepdaughter, Miss Stoner, is Dr Roylott’s violent past-
he beat his native butler to death and narrowly escaped a capital sentence.
Dr Roylott was imprisoned and greatly embittered by the experience. He married Miss Stoner’s mother and was content to live on her inheritance. After the death of his wife, Miss Stoners mother, he became more aggressive: coming to the attention of the police in fights with the neighbors-
he is a man of immense strength, and absolutely uncontrollable in his anger.
Due to his affinity with the tropics, the doctor also kept exotic animals on the premises which made the Stoner girls nervous –
...the doctor kept a cheetah and a baboon. We had no feeling of security unless our doors were locked.
Dr Roylott has already been violent towards Miss Stoner, as Holmes perceives-
Five little livid spots, the marks of four fingers and a thumb, were printed upon the white wrist.
He visits Holmes to warn him off the investigation-
Don't you dare to meddle with my affairs
When investigating Roylott’s affairs, Holmes also finds out that the inheritance from his wife will pass to their daughters on their marriage, which explains why they are in danger when their engagements are announced-
he has the very strongest motives