My theory.
She wants the Jailor dead, but can’t do it on her own. She lacks the power and army to do it. So she allies herself with him, giving him the souls he wants in exchange for the power she will need. All the while making the denizens of Azeroth hate her. When she finally has the power, she destroys the helm, opening the veil and goes in. Guaranteeing that the armies of Azeroth will follow her and kill the Jailor for her.
Thus completing her vengeance and freeing the dead from their prison in the Maw at the hands of the Jailor.
It’s just a theory, we’ll see.