After fighting their way into the Red Mountain, Nerevar and Dagoth Ur finally reached the Heart of Lorkhan. Nerevar initially did not know what to do, but Dagoth Ur insisted that it be destroyed immediately, trusting none to be exposed to it. Nerevar refused, and left him to guard it while he consulted Vivec, Almalexia and Sotha Sil.
Now what happens next is very important. The Tribunal decide to destroy it, but we now know that Almalexia, Vivec and Sotha Sil had no intention of doing that.
However, as they were gone, Dagoth Ur was left alone with the Heart, something that could grant immortality. He knew the tribunal would kill him and use it, and he knew that it was only a matter of time before they arrived.
So what would you have done, in his shoes? Destroyed it? Maybe he tried to: All we know is that he "expirimented" with it. Maybe he inadvetantly tapped into it, or maybe he took immortality to ensure that he could guard it from them when they arrived, and it actually seems likely that he did the latter. When they arrived, he refused to allow them near the Heart, saying that he promised Nerevar he would protect it from being misused.
Here is where it gets sketchy. Some say that Dagoth Ur thought Nerevar was corrupted, others say the opposite, and still others say that Almalexia, Vivec, and Sotha Sil assaulted both of them, but the story ends the same way. Nerevar dies, Dagoth is thrown into the fire, and the Tribunal misuse the Heart.
Dagoth Ur was right. The Tribunal did betray Nerevar. And for keeping his promise, he was hated and scorned by the Dunmer, while the traitors of the Tribunal were revered as gods. This is where Dagoth Ur, pissed off at his own people for betraying him, summons the blight, and fights ALMSIVI away from the heart, refusing to let them use it ever again.
To me, Dagoth Ur is just as evil as Azura is. She sent the Nerevarine to destory the Heart, which she knew would cause the Ministry of Truth to destroy Vvardenfell and much of Morrowind, because the Dunmer betrayed her. Dagoth Ur, angry because the Dunmer betrayed him, punished them by summoning the ash storms, and eternally guarded the heart from ever being used again. In fact, one could argue that Azura is more evil for what she did.
I'm just saying that Dagoth Ur was betrayed by his friends and his people, and sought revenge. Azura and the Nerevarine did the exact same thing, and yet people call her a "good guy" and Dagoth Ur a "bad guy".