The film Society (1989) was very sick and disturbing. These days the effects do look a bit lame but the horror of the story and what the people do to those not in 'society' is really sick.
This film takes incest and cannibalism to a whole different level.
I watched it when I was 12 and it freaked me out - it's maybe not that scary as an adult, but if you watch it alone or unprepared then it will certainly freak you out.
On the plus side it stars a young Billy Warlock - who I think was a bit of a dish