This movie is not for everyone. It's dark and brooding with some black comedy drizzled in. I've seen it compared to Heathers and American Psycho, but those didn't have the flavor of this movie. This felt more like Girl, Interrupted to me. A much more gritty dramedy about anti-social behavior.
Thoroughbreds is a dark movie about two rich girls that stopped being friends until recently and how their lives get twisted back together. It's more of a character study than a plot-based movie, so there are a lot of scenes of introspection, making it pretty artsy. One girl seems 'normal', but she's detached and self-important. She finds it freeing to be with the other, who is much more honest, but also a sociopath with no real emotions.
While the movie is a bit slow-moving, the pacing still felt right to me. And the music was all messy and crazy, yet it had purpose. It's a bit of a cautionary tale about why you shouldn't let your kids be secluded and anti-social. Learning right and wrong is a pretty essential quality for well-rounded people. Teaching that often requires access to many types of society, not just one. Empathy needs an understanding of different outlooks, not just your own.