What she is doing has nothing to do with 'fixing' anything. She is trying to destroy the cosmos, because it is self-righteous and uncaring. How ironic. ^^ She is not a god indeed, she is simply the tool that the Jailer needed to finally make his move. For this she has made bargains, accumulated (and continues to seek) power, made promises. She thinks the Jailer shares her sentiment that the 'ladder of lives' has to be dismantled all at once, but we already know what he wants is the same thing Sargeras wanted, the world soul under his control. And as I said, I think once she finds out that he has no intention of keeping his promises, she'll try and turn on him, which will fail, because all of her borrowed power will be gone and she'll just be a Banshee again, with not even Nathanos to care about her anymore.
And I disagree that Illidan was in any way shape or form redeemed in Legion (or his book). He is still just as bad as he was and we were even shown how oblivious he was to his own misdeeds and how he thought people should have praised him for what he did to them. He is an a**hole through and through, with only one factor that made him our ally this time instead of our enemy: he tries to save Azeroth.
The deciding thing, as usual, being: what are you fighting for. Revenge and annihilation or saving something/someone?