Did you miss the entire point of the Orcs literally being cursed by the Fel? They no longer had free will as long as the puppet master willed it. Gul'dan showed how he could destroy any Fel infused Orc, which is why they all feared him at the end of the film. The tragedy of the Orcs is that they cursed themselves because they believed in Gul'dan, which was a grave mistake. This one mistake haunted their people for generations.
Also, did you think the Fel corrupted Medivh had free will as well when he let the Orcs into Azeroth?