S-21, la machine de mort Khmère rouge (2003)
http://www.imdb.com/title/tt0368954/Well-done, but as some of the IMDB posters note, it assumes the viewer already knows something about the Khmer Rouge and S-21. Sounds like you do, so you might...well, "enjoy" sure isn't the right word. Worth your time, I'll say.
I looked up the movie after I read Philip Short's biography,
Pol Pot: Anatomy of a Nightmare. Also well worth reading.
Since I am one of those annoying
Fundamentalist Atheists, it was amazing to learn how big a part religion played in the insanity of the Khmer Rouge. Pol Pot was a Thedavara Buddhist, and even spent some time as a monk in his youth. So were many of his cronies in the KR top leadership.
Westerners were puzzled when the KR emptied out Pnomh Penh, but Short pointed out that this was exactly in line with Pol Pot's early religious training. That branch of Buddhism uses "country" and "city" as symbols of good and evil.