Syrian Orthodox bishop and another beheaded in Syria

U.S. backed Syrian rebels beheaded a Syrian Orthodox bishop with another man, likely the bishop's assistant though not yet identified. Metropolitan Paul Yazigi, the brother of Patriarch John, was killed by Islamic fundamentalists who cheered that they killed infidels.Metropolitan Paul Yazigi and Archbishop Youhanna Ibrahim were kidnapped by Chechen militants in April.A news report is here, the video at the end of story shows the beheading.Why is America considering to arm these cruel and barbaric people?