A federal jury yesterday sentenced a convicted killer to death for stabbing a 19-year-old single mother 82 times in her bathtub and leaving her infant daughter to die, bucking a national trend and marking the first time a jury has voted for capital punishment at the Alexandria federal courthouse. After less than two full days of deliberations, the 12-member jury said Thomas M. Hager should die for the 1993 slaying of Barbara E. White, who was beaten before being repeatedly stabbed in her Fairfax County apartment. As White begged for her life, her 13-month-old daughter toddled in the next room....