What is Roylotts motive for the crimes and why does Roylotts plan backfire?

Dr. Roylett's motive was money.  His wife's will stated
that the total income they would get from her estate would be 100 pounds.  Due to the
fact that agriculture prices had fallen, the estate was now worth about 750 pounds. 
Upon marriage, according to the will, each girl would get 250 pounds.  That decrease in
his worth would "cripple him to a serious extent". 

The
reason Roylett's plan backfired was because of the genius of Sherlock Holmes.  Holmes
found the bell pull that was connected to the ventilator. The ventilator had no purpose,
opening only to the bedroom next store instead of to the outside air. He noticed that
the bed had  been fastened to the ground so that it could not be moved.  He figured
something had to crawl down that rope.  When the snake crawled down the rope, Holmes
attacked it with its cane, sending it back to through the ventilator.  It was so angry
that it attacked the first person it saw, which was Dr. Roylett.  The snake he had
chosen was the deadliest snake in India and killed within ten minutes of being the
victim being bitten.