I have seen lots of disturbing movies in my 48 years. Here are a few that I feel stand out that I don't believe have been mentioned in this thread so far, in no particular order.
American Mary
From IMDB: The allure of easy money sends Mary Mason, a medical student, into the world of underground surgeries which ends up leaving more marks on her than her so called 'freakish' clients.
http://www.imdb.com/title/tt1959332/?ref_=fn_al_tt_1
Last time I looked it was on Netflix.
Das Experiment
Pretty unsettling considering it was based on the Stanford Prison Experiment. The 2008 "remake" was not nearly as good.
From IMDB: For two weeks, 20 male participants are hired to play prisoners and guards in a prison. The "prisoners" have to follow seemingly mild rules, and the "guards" are told to retain order without using physical violence.
http://www.imdb.com/title/tt0250258/?ref_=nv_sr_2
Red White and Blue
From IMDB: An HIV positive woman's quest to infect as many men as possible has gruesome consequences after she attracts the attention of a psychotic former Army interrogator and an emotionally fragile young man caring for his ailing mother.
http://www.imdb.com/title/tt1465505/?ref_=nv_sr_1
Kynodontas aka Dogtooth
From IMDB: Three teenagers are confined to an isolated country estate that could very well be on another planet. The trio spend their days listening to endless homemade tapes that teach them a whole new vocabulary. Any word that comes from beyond their family abode is instantly assigned a new meaning. Hence 'the sea' refers to a large armchair and 'zombies' are little yellow flowers. Having invented a brother whom they claim to have ostracized for his disobedience, the über-controlling parents terrorize their offspring into submission. The father is the only family member who can leave the manicured lawns of their self-inflicted exile, earning their keep by managing a nearby factory, while the only outsider allowed on the premises is his colleague Christina, who is paid to relieve the son of his male urges. Tired of these dutiful acts of carnality, Christina disturbs the domestic balance.
http://www.imdb.com/title/tt1379182/plotsummary?ref_=tt_ov_pl
Bully
From IMDB: Based on a true story. Naive teenagers plot to murder one of their own, who has been too much of a bully to them.
http://www.imdb.com/title/tt0242193/?ref_=nv_sr_1
Alpha Dog
From IMDB: A drama based on the life of Jesse James Hollywood, a drug dealer who became one of the youngest men ever to be on the FBI's most wanted list.
http://www.imdb.com/title/tt0426883/?ref_=nv_sr_1
Kids
From IMDB: A day in the life of a group of teens as they travel around New York City skating, drinking, smoking, and deflowering virgins.
http://www.imdb.com/title/tt0113540/?ref_=fn_al_tt_1
Excision
From IMDB: A disturbed and delusional high school student with aspirations of a career in medicine goes to extremes to earn the approval of her controlling mother.
http://www.imdb.com/title/tt1984153/?ref_=nm_knf_i2
Turn off the lights and start the show already!
opcorn: