And Then There Were 33: Towards Zero by Agatha Christie

Christie rarely deals in psychopaths, but when she does she tends
to handle them with panache, and this novel is no exception. (Not a
spoiler, as we are put on our guard almost immediately that a
psycho lurks in these pages....) While it is by no means one of our
absolute favorites, we quite enjoyed this one, which features a
number of familiar Christie tropes spun/twisted in ingeniously dark
ways.