Reading progress update: I've read 153 out of 325 pages.

This is a somewhat contrived story. And I am saying that as a regular reader of plots that are riddled with convenient twists and dreamt-up coincidences.
Anyway, the likelihood of a demented serial killer on an island that preys on only a single group of people, and no police having made any appearance at all yet, not even a single village bobby, ... even I'm going to say that this is a bit far-fetched.