Who was Dr. Roylott in "The Adventure of the Speckled Band"?

Dr. Roylott is Helen and Julia Stoner’s stepfather and Julia’s murderer.

Helen Stoner hires Sherlock Holmes to find out what happened to her dead twin sister.  She says that her sister said something about a speckled band when she died, and that she remembers hearing a whistle.  Holmes finds the whole thing meaningful enough to take the case.

Helen tells Holmes all about her evil stepfather, Dr. Roylott.  He is a quarrelsome and violent man....

Dr. Roylott is Helen and Julia Stoner’s stepfather and Julia’s murderer.

Helen Stoner hires Sherlock Holmes to find out what happened to her dead twin sister.  She says that her sister said something about a speckled band when she died, and that she remembers hearing a whistle.  Holmes finds the whole thing meaningful enough to take the case.

Helen tells Holmes all about her evil stepfather, Dr. Roylott.  He is a quarrelsome and violent man. She claims he has gotten worse.  He was not able to establish a medical practice.  Now he is the scourge of the neighborhood.

Violence of temper approaching to mania has been hereditary in the men of the family, and in my stepfather's case it had, I believe, been intensified by his long residence in the tropics. A series of disgraceful brawls took place, two of which ended in the police-court, until at last he became the terror of the village …

Holmes suspects Dr. Roylott immediately.  The man shows up as soon as Helen leaves and tries to threaten him, seeking to impress Holmes by bending a fireplace poker.  Holmes bends it back easily, and does not consider the show of strength anything more than information to enlighten him about the man’s character.

Dr. Roylott’s home is full of odd creatures.  He keeps wild animals, like a cheetah and a baboon.  Of course, the most exotic and dangerous of these is the murder weapon, his swamp adder.  Holmes comments that a doctor is the worst sort of criminal.

“Subtle enough and horrible enough. When a doctor does go wrong he is the first of criminals. He has nerve and he has knowledge. …”

Doctors are supposed to help people, not use their knowledge to kill.  Dr. Roylott definitely failed to live up to his oath.  He killed one of his stepdaughters and tries to kill the other just to steal their inheritance.  Unfortunately for him, his murder snake turned on him.