Even if a mortal can't understand it, and it doesn't make sense to us, Mehrunes Dagon will destroy everything. It's what he does, he can't help it. He won't try to preserve a tower, even if it is his. that's what I get from him, anyway.

Just like Azura must end and begin everything. So whenever you're in the middle of something, if she is around, she is compelled to end it and begin something else? Just as Sheogorath must gnaw at the stability of creation every single moment of his existence?

If Dagon had to destroy everything, then he'd be quite alone. I mean, his whole covenant with the Dremora would be broken and he wouldn't have his armies. After all, would you want to keep working for a guy who whenever you met him, he destroyed your physical body, sending you into the void only to return a short while later? No. That's silly.

You know, actually... It makes no sense whatsoever. Dagon is a nice guy (leper king), but was cursed to destroy Nirn. Surely, he has no reason to cause destruction in his own realm? His realm should depict the personality in the leper king. Just because he's destroying Nirn, doesn't make him an angry "must destroy everything" kinda person, does it?

Makes me think of the age-old adage to "Rise From The Ashes". The power of destructiveness will inevitably give rise to a new beginning.

He ended the line of the Septims, destroyed the souls of every past ruler of Tamriel (they were in that amulet), and forced a change upon the whole world. Through those actions he also (indirectly) effected all of the events that played out in The Infernal City. He essentially started a new age. Perhaps he was the one who kidnapped Vivec so that Vvardenfell would be destroyed, it was definitely him that started the Oblivion Crisis. He started a new era and a new age, quite possibly entirely different than one we've ever read of before. If he symbolizes revolution and change, and he was worshiped by the Mythic Dawn (Rising of a New Age) who were the opponents of the Lineage of Septims (Who've been in power for a long amount of time), isn't that a clue to the direction history would be headed?

I don't think he necessarily wants Nirn, he's done what he was destined to do. He couldn't have killed all of the Septims without being banished back, because then the world would have been completely destroyed, and without something to destroy, the concept of everything he stands for would cease to exist.

It is kind of like the Hulk:

Hulk gets angry, so he smashes.

Mehrunes Dagon gets cursed, so he smashes.

Plus, it was the opportune time for it.

Completely agree, Mehrunes Dagon is more than just pure destruction. He's also the prince of ambition and revolution - after all, it was ambition that drove Tiber Septim to conquer Tamriel, causing destruction, as an example. He is far more subtle than many think of him as.