I think you're on to something here, as the evidence we have to date implies that Hydaelyn only functions in a capacity to stop the final days from coming to pass. A permanent solution to what Zodiark can't fix. Balance is the desired goal.
Zodiark on the other hand, may not even be aware he is causing the thing he was summoned to stop. And if his volume of power is enough to end all existence and he blindly uses that power, he becomes like a child with nuclear weapons or, worse, an idealistic youngster who lacks the foresight to think about the consequences of their actions.