Why was Helen Stoner afraid of her stepfather in "The Adventure of the Speckled Band"?

Helen Stoner is afraid of her stepfather because he has a violent temper and her sister is dead.

Helen Stoner comes to hire Sherlock Holmes.  When a person comes to see the famous detective Sherlock Holmes, it means that the person is in serious trouble.  Helen’s trouble is that she lives with her stepfather, and he is not a good person.  The bigger problem is that her twin sister is dead and she is suspicious.

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Helen Stoner is afraid of her stepfather because he has a violent temper and her sister is dead.

Helen Stoner comes to hire Sherlock Holmes.  When a person comes to see the famous detective Sherlock Holmes, it means that the person is in serious trouble.  Helen’s trouble is that she lives with her stepfather, and he is not a good person.  The bigger problem is that her twin sister is dead and she is suspicious.

Helen tells Holmes that her stepfather Dr. Roylott is from an old family.  He was a doctor, but when he married her mother he no longer worked.  They returned to the old family manor, and his behavior changed.

But a terrible change came over our stepfather about this time. Instead of making friends and exchanging visits with our neighbours … he shut himself up in his house and seldom came out save to indulge in ferocious quarrels with whoever might cross his path. 

The violent quarrels with neighbors are just the beginning of Helen’s worries about her stepfather.  Helen had a twin sister, Julia, but she died under suspicious circumstances after they moved.  Helen is worried that strange things are still happening at her house and she might be next.

Helen is right to worry.  Holmes and Watson go to Stoke Moran to investigate. They stay the night, and hide in a room waiting for something to happen.  What happens is that a snake comes into the room.  It is a very deadly snake called a swamp adder.  It is what killed Julia.

Holmes explains to Watson that Dr. Roylott was dangerous because he was a doctor.  Doctors are supposed to do no harm, but this one did not stick to his oath.

When a doctor does go wrong he is the first of criminals. He has nerve and he has knowledge. Palmer and Pritchard were among the heads of their profession. 

The snake turns on Dr. Roylott and kills him, so from that point on Helen no longer needs to worry about her stepfather.  She can be safe in her own home, as long as she gets rid of all of the roaming animals!