In this story Ripley incarnates the perfect sociopath - a person who conforms to society's rules and looks good on the outside but who is perfectly corrupt and without moral checks underneath. To a sociopath there is no right or wrong; he even takes a kind of perverse pleasure out of committing a crime and then getting away with it "scot free."  The sociopath pits his intelligence and cunning against society's barriers (rules of behaviour and laws) and tests his ability to foil others. Of course he does this for his own benefit or profit, but his real motivation is the pleasure he gets by playing his "game" and winning.  This is the personality profile Highsmith portrays through the 'perfect villian' in this story.

Apart from just the thrill of scaring her readers to death, perhaps the author would advise them to be on guard against this type of deviant personality since more than just a few are running loose, just looking for the chance to do somebody in.