Islamic State fighters have killed scores of people and are using tens of thousands more as human shields as government and Kurdish forces battle to reclaim Mosul, United Nations human rights investigators reported on Friday.After killing a group of 25 former servicemen on Tuesday, the militants shot to death at least 190 former members of the Iraqi security forces at a military base in Mosul on Wednesday, Ravina Shamdasani, a spokeswoman for the United Nations human rights office, told reporters in Geneva.