Request #613 From Stefani

In the mid to late 90s, I read a book from the library that was told in first person. A group of teen friends is taken to an island with the boat to return in like 5 days to pick them back up. One by one, the start dying until the last day when there were 2 left, the narrator and another girl. They end up coming face to face outside. The narrator has this mental monologue about what do you say when you are confronted by a killer. Turns out the narrator had worked her way into the friend group to get revenge for the death of her friend and was the one who was killing everyone.