The Dark Side of Woodstock? The death knell of ’60s counterculture? The End of peace and love? Pretty much. Maysles-Zwerin’s now-classic documentary about the 1969 Rolling Stones American concert tour inadvertently captures the darkest side of the American psyche as a murder takes place during a free performance at Altamont Raceway, CA. Essential, but let’s face it, a bummer.