Props to the movie's Wikipedia page for spoiling the ending in the above-the-line plot synopsis (it's been edited now, thankfully).
Thoroughbreds is a cult gem in the making; dryly funny, patiently shot and terrifically acted. A few scenes perhaps drag on a touch too long, but its minimalism often lends the film a quiet eerieness.
RIP Anton Yelchin of course, especially this being his last release I'm likely to watch. Taylor-Joy and Cooke were great as expected.