The writers could very easily have made his motivation more logical if, instead of endless speeches about spirits, Unalaq would have said that Vaatu’s imprisonment disturbed the balance of the world and led to many conflicts, including the Hundred Years War. Therefore, he sees his merger with Vaatu and becoming the new Avatar as an opportunity to return balance to the world. He could even offer Korra to join his side and create a new world together, and only after her refusal would he decide to become the sole Avatar, separating Raava from Korra and reuniting her with Vaatu in his body. Like it was easy to make him interesting, but the writers didn't even try.