The attack on Darkshore was plotted by Sylvanas, the whole supposed plan of action was suppose to capture and occupy Teldrassil as bargaining chip to get the Alliance to stand down. Of course he was killing night elves they were the enemy but the burning of Teldrassil was not apart of the plan. The whole Horde didn't have a clue that Sylvanas was going to do that. Also Saurfang isn't going to marry a genocidal corpse.