What is the motive for dr roylotts not wishing his stepfather ever to marry?

Dr. Roylott's motive for not wanting his stepfather to remarry stems from financial greed and a desire to maintain control over his inheritance. He stands to lose a substantial portion of the family fortune if his stepfather marries, as his wealth would then be shared with a new spouse. This selfish concern drives him to act ruthlessly, as he seeks to eliminate any threats to his financial stability.