So, since a Daedra (Mehrunes in Oblvion) suffered a defeat Alduin/Akatosh (an Aedra) must also be defeatd? interesting theory
Dagon just wants to destroy all those places he hid from being eaten the previous kalpas, and stuck onto the world in new kalpas. To make them bigger, so the eating takes longer and he has longer life-time.
That plan backfired, so he was cursed to become Mehrunes Dagon and try and undo the works he made as a leaper demon.
He just wants to jump again.
This would mean that Alduin/Akatosh (seemingly) worked against himself/ his purpose. But that should be no great mind boggler.
After all, our own christian culture has something very much like it in the form of the devil.
It goes like this:
1) God is the greatest, there is nothing greater than God.
2) Therefore there is nothing outside God. If there would be then something would be greater than God, namely God + that outside him.
3) Everything is part of God.
4) Therefore the devil is a part of God.
5) The devil is a part of God that to our limited understanding seems to strive against itself.
So a god that (to our eyes) tries to undo his own works is perfectly logical, from a god-place point of view.