SELMER, Tenn. - Mary Winkler was the quiet, unassuming wife of a small-town, by-the-Bible preacher, seemingly devoted to church and family. But now her husband, Matthew, is dead and she is charged with shooting him in the back with a shotgun. Authorities won't discuss a motive, and church members say they didn't see any indication she was unhappy. But experts say preachers' wives often struggle with depression and isolation, expected to be exemplars of Christian virtue while bearing unique pressures on their private and public lives. Gayle Haggard, author of "A Life Embraced: A Hopeful Guide for the Pastor's Wife,"...