What if Ahriman is not a bad person, nor has dark magic twisted his mind. His loyalty to Sauron is just a show.
We can see this clearly in two simple ways: First, he saved Anaynah – something someone truly on Sauron’s side would never do. Second, Anaynah herself has wondered if he hides other reasons for his actions, because she has never seen any signs that dark power has changed him.
What his real plan is, we do not know. But these small, clear facts tell us he is playing a part to reach some goal that has nothing to do with helping Sauron rule or destroy.