Noah had played by the rules for too long. Heaven and Earth needed help, and he was willing to aid them as long as benefits came in his way. However, the environment on the other side of the Immortal Lands was too hateful.
Heaven and Earth were basically asking Noah to help his enemies building an army. His patience had a limit, and the last interaction with the natives had triggered his anger.
Noah was in an environment invaded by enemies of Heaven and Earth. Helping the Devils and the mutated magical beasts would benefit him in the long run, but his companions always made sure that he didn't cause any mess.
His knowledge about that side of the Immortal Lands had always been too poor to evaluate the whole higher plane. After all, his side also had fanatics and zealots. It would be unfair to judge the entire political environment after knowing only a few forces.