The Sixth House was about as deranged as it gets. Deformed, too.
I dunno, Mankar and his group of cultists are pretty much trying to accomplish what Alduin aims to do, only they used Mehrunes Dagon as the vehicle of destruction instead of Alduin. This is likely due to the Covenant still being intact, preventing the Preserver act as the Destructor, and that Mehrunes Dagon has a reason to destroy Mundus, other than he's the prince of destruction.
With Dagoth Ur, he saw the false tower, and thought it true. The dreams of a dead and mad god was made manifest, and desired to spread his nightmare he mistaken as a dream through the second Numidium.
As for Mankar's speech, he was somewhat correct. Mundus is the realm of Lorkhan, but it was made for mortals, not his own selfish desires (unless you were to ask a mer). He's half correct that Mundus is Mehrunes Dagon's birthright. The part he missed is that it's Mehrunes Dagon's birthright to try and completely annihilate it. And to the mer, by destroying Mundus, he'll liberate the mer from what they perceive as a prison, and the world will be sent back to the Dawn. His beliefs, while having some mannish tones to it, aims for what most mer desire, the destruction of their prison, Mundus, and to be liberated and go back to the Dawn Era.
Personally, I find the mer to be nothing more than the old geezer on the corner of the road who still complains about gas being two pennies. They want to go back to what they believed to have been a sweet and awesome past, and are consumed with false notions of nostalgia. They refuse to go forwards, and that shall be their undoing.