I really do wish the story had been different. Instead of making tagah a one-of-a-kind villain the cooler explanation of why he uses air bending in such an offensive capacity could have been that avatar Sonam established the air bending empire and Tagah was her acolyte. It would have made Aang reconcile his understanding of air nomad culture with the truth that in the past, air benders had been imperial conquerors. A lot of interesting story in that direction I think. And you can explain Sonam locking Tagah in ice by initially saying that the world has united to destroy the air benders, but the gaand gradually find out the truth - the world United in self-defense against the air benders.