I would also agree that it sounds like a story made up to frighten small children! He wouldn't have had the opportunity to be a "serial" rapist as he would have been dealt with the first time - probably with a sentence of death.
The first convicts with that surname were William and Edward RAPER (possibly related)of Northallerton who were tried for larceny at the York quarter sessions (Yorkshire)in April 1817. They received a seven year sentence.
I suspect that William did not make it to Australia as I cannot find anything more about him after his time on the hulks in England, but Edward arrived in NSW on the ship "Tottenham" in January 1818.
There was also a John RAPER who was tried at Middlesex for picking pockets and arrived in Tasmania in July 1824 on the "Phoenix" (2). He was given a life sentence and received his certificate of freedom in 1849.
There are a few later convicts and also many free immigrants named RAPER who arrived in the 1800s from both Ireland and England.
As Marg and Jean have said, just trace your family backwards and you will eventually have the right story.