Yah - I'm at that point now in my playthrough. When you look at the eye of the moon, Batlin says in the cutscene "When I open the Wall of Lights, I'll be more powerful than the Guardian himself!" Of course, before you confront Batlin, Selina says "With the Guardians help he (Batlin) and I shall meet in the Eternal Void to conquer new realms together" underscoring that Batlin's lying to everyone - I suppose he suspects the only way to stay alive is to become stronger than the Guardian.He believed that getting into the Void allowed him to absorb unlimited power. He never did explain how or why.So what was Batlin going to do? Kill the Guardian? Usurp his power?
Batlin further says before his demise "Now I shall enter the Wall of Lights and become immortal! Then I shall return to destroy thy mortal soul!" then later "I have been tricked! Avatar! I beseech thee! Help me before..."
Then the Guardian states "See how I reward those who fail me!" or such.
So, the take is that Batlin failed at bringing the Guardian into the world in U7. Somehow, Batlin's whole ambition was to become greater than the Guardian. Effectively, the Fellowship has been disbanded, Batlin failed, and Batlin is led to believe by opening the wall of lights he can become immortal/tap into unlimited power or such. Unfortunately he uses the wrong Blackrock serpent - even though the impression in the ground clearly shows the opposite serpent.
So, I think Batlins ambition was a ruse, and Batlin's story is told in parallel with the imbalance in the land, not the cause of the imbalance in the land. The Avatar restores balance to the land, but the Guardian pulls the Avatar into another world.