“There is no sin. A man does as he pleases with his property,” the most brutal of the slave owners, Edwin Epps (Michael Fassbender), blithely declares after an act of extreme viciousness late on in Steve McQueen’s Oscar front-runner, 12 Years A Slave. His choice of words is telling. It points both to what makes the film so powerful and to why it often seems so one-dimensional as drama.