How does Roylott's plan backfire in The adventure of the speckled band.?

In "The Adventure of the Speckled Band," Dr. Roylott's plan backfires when his deadly snake, intended to kill Helen Stoner, is thwarted by Sherlock Holmes. The snake, a trained Indian swamp adder, turns on Roylott instead when he attempts to summon it back to him. This unexpected turn of events leads to Roylott's own demise, exposing his murderous intentions and allowing justice to prevail. Holmes's quick thinking and observational skills ultimately save Helen and unravel Roylott's sinister scheme.