Generations of mob rule are undone by a standard-issue revenge tale in “ Violent Ends ,” a sparing but basically styleless thriller that chronicles a crime family’s unremarkable infighting in the boondocks of Arkansas’ Ozarks. For his long-in-the-works sophomore feature, writer-director John-Michael Powell maintains a likably low-key interest in the local flavor of his home state, but it’s small potatoes in terms of personality. His self-serious approach proves a terminal match for his crime