It was clear that his enmity with the Goddess of Nature started with her Brother's death. For her, thousand years of solitary confinement was not even worth mentioning, compared to her loss of her only family member at the hands of her so-called friend.
If he wanted to turn her around, he had to first return her brother and start from there.
Fortunately, it was easier to bring back the brother of Goddess of Nature since he didn't have to go to the Nether Realm to bring his soul back. Her brother's soul was already inside his Grimoire after all, with the consciousness intact. All that was needed was to create a body, which was even easier for him.
"Why are you doing it?" The Goddess of Nature frowned. "Instead of just killing me, why are you freeing me? And even help me bring my brother back?"