A highlight of last year's Cannes Film Festival and the recent New York Film Festival, Rithy Panh's moving documentary bearing witness to Cambodia's Khmer Rouge era now makes its much-anticipated Bay Area debut. Out of 17,000 individuals imprisoned and interrogated at the notorious Phnom Penh camp Tuol Seng, only three people survive today. Panh returns with two of them to meet the “other side,” their former guards. With the tables turned, the two survivors, one a writer, the other an artist whose remarkable paintings lend both reality and nightmare to the film, become questioners, asking those who once interrogated them what caused them to kill. “You killed without humanity,” they say to one former jailer, who merely responds, “I was young.” In a manner so unemotional as to be beyond horror, the guards explain how prisoners were tortured and killed, read from their “interrogation journals,” and reenact the routines of atrocity for a camera that becomes not only recorder, but witness. An elegy in the form of a film, S-21 is “unforgettable,” wrote J. Hoberman in the Village Voice, “as horrific an exposure to evil as Shoah.”