Post VI and the last 2 paragraphs of post VII have the answer to your question.
But in short she was a fragment of the Goddess' soul, the more megalomaniac and power-hungry part of it. In her final years the Goddess excised that part of her soul so that both parts of her could try to find peace before she passed away. But, separated from the whole, the "evil" part of her soul took the form of a demon and had its own machinations, she wanted to possess the MC so that she could have a physical form and try to attack what remained of the Goddess and usurp her power.
