Dr. Roylott's personality is detrimental to the success of his objective, to murder his stepdaughters. Only reading the first few chapters of the story, the author wrote him as a person with an intimidating, violent, and an enclosed persona.
In both his past and present, Dr. Grimesby was a man of violence. In addition to his violence, he also happened to be out of control and impulsive when he was angry. With his past delinquency, it showed not only that he had the capacity to kill (murdering his butler), but he did it without thinking it through and on impulse (just for a robbery). Making him a main suspect for the crime that he was planning to get away with.
Mr. Roylott was also quite clever and secretive.