Eden Lake (2008... Lea's suggestion): Chavs and their families being nasty, creepy cunts. At least that's how she says it goes.
Harry Brown (2009): Michael Caine doing his best Eastwood in Gran Torino impersonation, yet actually taking the worst people out. Again, this includes chavs.
Das Experiment (2001): German film loosely based on the Zimbardo Stanford prison experiment. You'll definitely be creeped out how "normal", rational humans turn into complete monsters in little time.
Audition (1999): Two words... TAKASHI MIIKE. Whenever he directs anything, it'll be freaky. Many of you have likely seen this and can attest to it. Essentially, a widowed middle-aged man conducts fake "auditions" to find a mate and believe he's found love. Oh no, buddy. You found something else.
Sin Nombre (2009): A really well-crafted tale about a girl wanting to illegally immigrate into the US. Meanwhile she reunites with her father and has to fend off a pretty violent gang.
City of God (2002): More gangs, but cooler ones (i.e. Brazilians). Takes place in Rio and follows the lives of two childhood friends who take different paths. Pretty intense.
I Saw the Devil (2010): KOREAN REVENGE FILM. That's all you need to know. It must be seen to be believed. It's going to be released in the States soon but uhhh, why wait? Torrent that bitch.