Let's stop pretending that the horde's reasons were as clear as day.
-The archers on the ridge, at the sound of the horn, didn't even try to GESTURE the alliance that the horde forces were in disarray. They just walked away with a smug look on their rotten faces.
-The alliance could have no idea that the horde was having so much trouble, and a mere horn couldn't signal it. As I said, they didn't even try to tell the alliance.
-The horde preferred rescuing random horde members than sparing a flying thing for Varian. After what he's done for them in Siege of Orgrimmar, the horde should have tried their best to prove they can be allies instead of repeated betrayers. But nooooo.
-The next time you see Sylvanas she's attempting to enslave Eyir and bargaining with a cruel deity of the underworld. If you were Genn, would you believe it if someone told you "they had good intentions I swear!".
Let's be real.