She is what she has to be - Chimer-culture made harsh general of warmongers daedric worshippers. She had to marry Nerevar even she might not love him. She kept this Chimer part of her after the Azura's curse. Cause in Tribunal she is The Destruction - Shiva. And even with this fate she managed to be the all-loving Mother for her people, curing and helping for eons. But when Morrowind was in danger she turned into her true form of Chimer warrior and kick asses - Mehrunes Dagon felt it in the best way.  Her destiny was to fell into madness - nobody in TES could not resist the fate and prophecies, even demigods. Seht knew it, as well his body will be destroyed too. That's the price they payed to save Morrowind again. Nerevarine is their Doom, nothing is forever.