Ripley had nothing to do with it.
Dickie Greenleaf did it, killed Peter, killed Freddie, killed Tom Ripley in a little boat in the middle of nowhere. Why Peter? Because Peter thought he was Tom and would have given it all away.
It almost worked.
Except Dickie Greenleaf is months dead and Tom, Tom's on a boat to Greece now with a dead man and a girl who called him Dickie.
He was only trying to be happy. He couldn't stay Dickie because that was dangerous, but he was trying to be happy. He had Peter and Peter cared and maybe understood, and he'd tried so hard to be happy. Peter had been. Peter liked Ripley.
But Tom didn't like Ripley. Tom liked Dickie (far too much) and Peter (not quite enough), but knew too much about Ripley to really care for him. Tom is common, Tom is no-one ('Tom is talented; Tom is tender,' Peter had said). Tom must be mad. And Tom is going to be caught now.
He wasn't talented at happily-ever-afters. He'd had that, maybe, with Peter, and had had to throw it away because Meredith and her family were there and she called him Dickie and -
A few hours as Dickie Greenleaf before the boat landed and they found Peter and started asking questions. A few hours...
Stupid, stupid mistake, Dickie. Or had it been a mistake at all? Why had he given Meredith that name? And why had he learnt Dickie's voice, movements, style, if not to become him? Tom thought that he had really loved Dickie, been close to Dickie; he couldn't have been so calculating, so cold, he wasn't like that at all -
Except, wasn't it amazing luck to look like, sound like, act like Dickie after it was done? And wasn't it interesting how Tom had felt so compelled to confront Dickie out alone like that and he must have known how Tom felt before; and how Dickie hated confrontations, which Tom knew, and also his inevitable reaction; and -
But no, it was not interesting in the least because the problem was really how he had reacted when trapped on a boat with many people who thought he was Dickie and one person who thought he was Tom and so sorry Peter, I couldn't kill them all and didn't know what else to do. A few hours...first to dawn, then to dock, then to death - Peter would not really be dead for Tom until he was found.
Such a futile death for Peter, who was really too good; could he ever understand that need to be someone else? No, Peter wouldn't have understood, because he liked Tom, why Tom needed so badly to be Dickie. Dickie had seen, and was disgusted by it, and now Tom had to answer for them all - Dickie, Freddie, Peter.
But if Peter weren't found? If he could get Peter overboard, convince Meredith to call him anything but Dickie (he'd need new papers again - Peter's?) maybe he could manage to not be Tom Ripley anymore. He had to act, and soon, to stretch out these last hours. He was talented at that, riding on an impossible situation until it changed for him. That much he could do, again and again. He just didn't know if he'd ever be able to stop. Tom closed his eyes and thought about what to do about Ripley.