In the early hours of Jan. 1, 2009, Oscar Grant III, unarmed and lying face down on a subway platform in Oakland, Calif., was shot in the back by a white Bay Area Rapid Transit police officer. The incident, captured on video by onlookers, incited protest, unrest and arguments similar to those that would swirl around the killing of Trayvon Martin in Florida a few years later. The deaths of these and other African-American young men (Mr. Grant was 22) touch some of the rawest nerves in the body politic and raise thorny and apparently intractable issues of law and order, violence and race.