The real question is; why did you give the star to azura? --- spoiler below, dont know how to make spoiler tag ---
For one you said yourself that you were a dark elf [censored]. So why not screw over azura, she plays around with people's lives like they're toys (Aranea for example)
And second, the Black star is way better then azura's star. With azura's star you gotta go kill things like mammoths for grand souls (not very hard). Whereas the black star, you can soul trap a level 1 [censored] bandit and still get a grand soul out of it.
I may be a Dunmer [censored], but I love my people and I revere the three good Daedra(Azura,Mephala,Boethiah). I kill, [censored] and pillage, but when someone hurts a Dark Elf, I turn into a Dunmer Paladin, and avenge or help them. I killed Nelacar for merely suggesting to betray Azura.