Dr. Roylott's motive was money. He was motivated by greed, specifically a desire to inherit the money from his late wife's estate. This inheritance was contingent on his stepdaughters, Helen and Julia, remaining unmarried.
Here's why:
* Financial Straights: Dr. Roylott was in financial trouble. He was a disgraced doctor, had lost his medical license, and was struggling to make ends meet.
* Inheritance: He saw his late wife's estate as a way to solve his financial problems.
* Marriage Threat: His stepdaughters' potential marriages would threaten his claim to the inheritance, as it was specifically written to only benefit him if they remained unmarried.
To ensure he got the inheritance, he resorted to murder. He tried to kill Julia with a venomous snake, and he likely planned to kill Helen too.
Let me know if you have any other questions about the story!