There is no excellent beauty that hath not some strangeness in the proportion.
-- Sir Francis Bacon
Tom Ripley, like many of Highsmith's characters, is a very credible sociopath, coldly capable of disposing of anybody who gets too close to his secrets. It is easy for him to kill, and the fact that he does so while living the haut-bourgeois life in an elegant French house adds to the fascination we have for him.
Of course we know that it all started with the murder of Dickie Greenleaf, and as we see his life unfold over the series of novels, we may cherish hopes that sooner or later Ripley's criminal past will catch up with him. But it does not, and after several novels I suspect that many readers are actually unwilling for that to happen. Why? Because we are fond of Ripley? That is hardly likely -- Tom Ripley may be charming and urbane, but he is not really very likeable.
Perhaps we merely want his story to continue because we are enjoying it so much. If Ripley had been arrested, or disposed of by somebody he had crossed, then that would have been the end of the series, and that would have been a disappointment. As it happened, Ripley survived his creator, and is still presumably living in Belle Ombre, his house in France, awaiting some author to approach the Highsmith estate with a request to continue to record his dubious doings.
-- from The Wall Street Journal