Well, it's clear that Hassildor has no love for vampire-kind (going as far as to exterminate those uncivilized vampires who near his domains). It could be that his benevolent spirit makes him decide to stop these vampires from becoming too powerful for mortals to handle...or he realizes that no sun means no crops, and no sun means progressively colder temperatures and both of those mean the eventual extinction of man and mer.
The greatest deception of all would be if Janus Hassildor founded, or supported the Dawnguard in his capacity of Count of Skingrad, hiding his vampirism even from the vampire hunters (if anyone could, it's Janus). Perhaps he feels that the only people who could threaten or kill him are other vampires, so he works to exterminate those others.
Interesting concept there maybe he does want to eliminate them before they get to powerful and extinguish the sun.
What if he has been biding his time to unleash these vampires and eliminating the sun has been his true motive all along and he was just waiting until he had these vampires under his belt, and he extinguishedtbe uncivilized vampires near him just to save face, this is speculative of course but what if his intentions are darker than what we have been lead to perceive.