ALSO SPOILERS:
Zôreth was well aware of how dire a situation the Stout-axes were in after Sauron devastated their halls, but if the Stout-axes had no use to her, she would have fallen in line with the rest of her kin. While it seems merciful from the perspectives of some of the Stout-axes, it was a self-serving act by Zôreth to gain power by leveraging the Stout-axes' desperation to aid in her own pursuit of power. In fact, her desire for power of her own was so overwhelming that even the Order of the Eye found her methods unscrupulous, selfish, and to some degree: blasphemous against Sauron himself. Zôreth is/was a pariah, even among her own people.
As for King Greytooth, he had a lot of things to reconcile with his newfound freedom. He had to at once transition from forgotten prisoner to a beacon of hope among his people, while at the same time battling the hold the Dwarf-ring had on him from years of torture by Sauron. Learning that he'd essentially been tricked out of his family heirloom by both Zôreth and Sauron at the same moment, he snapped in a fit of anger and despair and the Ring reasserted its lingering hold on him. Zôreth was evil, and held his people as slaves - alive, but a far shot from the glory of their past. King Greytooth wanted to break that cycle, and Gimli's better sense failed him in that moment as he was still caught between his duty and his personal interest in seeing the Stout-axes become an ally of his people.
In the end, Zôreth does predict King Greytooth's downfall in seeking the Ring, but she wanted that same power for herself and would not have hesitated to wield it were the roles reversed. If she got what she wanted, she would have cast the Stout-axes aside immediately.
I hope that helps clear it up!
Last edited by ThePinion; Dec 18 2017 at 04:03 PM.
Designer, Content
"A man that flies from his fear may find that he has only taken a short cut to meet it." - J. R. R. Tolkien, The Children of Húrin