So I watched the Roger Spottiswoode film version of Ripley Under Ground – co-written, incidentally, by Donald E. Westlake, alias Richard Stark – and it ain't that great. There are all sorts of problems with the movie – Barry Pepper's performance as Tom Ripley is decidedly lacklustre (physically he reminds me of a young Gary Busey, but with none of the intensity or magnetism or, apparently, acting ability); Willem Dafoe as art collector Murchison is pretty bloody awful – but the main issue is that Tom is effectively neutered. The decision was evidently taken to make Ripley Under Ground (the film) standalone, but the problem there is, by removing Tom's backstory, you get no sense of the awful things he did in The Talented Mr. Ripley, and his general lack of conscience in doing them. The best that Spottiswoode and co. can come up with for Tom in the movie is that he's a bit of a rogue and a bit of a chancer. That's not the Tom Ripley we know and love (or indeed loathe).
And so, logically, I can see why they also took the decision to make Murchison's murder... not a murder. In this film, it becomes pretty much an accident – because this Tom clearly hasn't killed before, so he has no impetus to start doing so now. He gets in a fight with Murchison when Murchison uncovers the forgery and deception at the heart of the story, sure, and he hides the body, but he doesn't actually kill him. In fact, Tom doesn't kill anyone in the movie. That's a major piece of the Ripley psyche removed right there, possibly even the most important piece: his willingness to kill to preserve his way of life, and his almost total lack of conscience about killing (with the exception of Dickie Greenleaf – maybe).
Once you remove that aspect of Ripley, that murderous thread of self-preservation born of the sense of entitlement he has for a better existence for himself, everything else falls apart, which is why the film doesn't work. In the end it's actually left to Tom's girlfriend-cum-wife Heloise to take up the potential criminal mantle, because now it makes no sense for Tom to do so (her eventual taking control of events is slightly foreshadowed throughout the film).
It's not all bad, mind. Ian Hart (who would go on to play Tom Ripley himself in a series of BBC Radio 4 adaptations of Highsmith's Ripliad) is quite good as painter/forgerer(erer) Bernard, and Tom Wilkinson is eminently watchable as the police inspector, Webster (Wilkinson is always great, as anyone who's seen his performance in Michael Clayton can attest); there's a nice bumbling sequence where the two of them are driving round the English countryside trying to find the house where Ripley, Jeff Constant and co. have hidden the body of Derwatt, the painter Bernard's been forging. But as a result of the meddling with Tom's character and motivation, what you're left with is a mildly entertaining but essentially toothless movie – and definitely not a genuine Ripley flick.