Late August is not considered the optimal time of year to open a collar-grabbing social-issue documentary. The Bay Area, though, is one place where conscience never takes a vacation. Annie Sundberg and Ricki Sterns tense, unforgettable The Devil Came on Horsebackdetails one mans experience in Sudans western province of Darfur, where in 2004 a local militia set about erasing the non-Arab population with the tacit approval of the Arab-run government. Brian Steidle, a former Marine captain, was working as an observer for the African Union as the genocide unfolded. In the right place at the right time -- actually, more like the wrong place at the wrong time -- Steidle photographed unfathomably gruesome atrocities. That might have been sufficient for some people, but not Steidle. He returned to the U.S. and waged a one-man campaign to galvanize media coverage and stop the bloodshed. The movie captures his painful odyssey, along with the hideous suffering in Darfur. Steidle bore witness and spoke his conscience; the least we can do is listen.