Dickie Greenleaf: You're so white! Have you ever seen a guy so white, Marge? Grey, actually.
Tom Ripley: It's just an undercoat.
Dickie Greenleaf: Say again?
Tom Ripley: You know a primer.
Dickie Greenleaf: That's funny. Margie likes that 'cause she's so white too.
Marge Sherwood: Yes, I do and you're not funny.
Dickie Greenleaf: "See Venice and die," is what they say? Or is it Rome?
Freddie Miles: Tommy. How's the peeping? Tommy, how's the peeping? Tommy. Tommy. Tommy. Tommy. Tommy.
Dickie Greenleaf: We're all only children. What does that mean?
Tom Ripley: It means we've never shared a bath. I'm cold, can I get in?
Dickie Greenleaf: No.
Tom Ripley: I didn't mean with you in it.
Dickie Greenleaf: Okay, get in. I'm like a prune anyway.
Herbert Greenleaf: You know, people always say that you can't choose your parents, but you can't choose your children.
Freddie Miles: In fact the only thing which looks like Dickie is you.
Answer: After running into Meredith, Tom's plan was to stay in the cabin with Peter the entire trip to avoid them seeing one another. But after Tom makes that suggestion, Peter tells Tom that he seen him kissing Meredith. He couldn't avoid or kill her since Peter already saw her. Family members also saw Tom with Meredith. If Meredith suddenly vanished or was killed I'm quite sure there'd be an investigation. Peter was the only option. No one is on board to report Peter missing. Once the boat docks, Tom will be long gone before they discover Peters body. Unlike Meredith, her family would be looking for her immediately since they're traveling together. I'm quite sure he'd much rather have killed Meredith, she meant nothing to him.