"It didn't have any other desires." It's the statement of someone who looked into a void and still has nightmares about how the void laughed back. "It had its own body and could bend people's minds to its will if they were of a similar inclination. And it was killing Cabeswater and my friends. It was consuming them." Gansey has studiously avoided the black goo because it reminds him so strongly of the oily miasma of the Demon's corruption inside Cabeswater. "The Demon was uninterested in coexisting with anything, and the woman who thought she could control it..." She died an ugly death. Gansey shakes his head as though to clear it. He's aware that he's not explaining this well, but it's sort of like trying to describe the worst night-terror someone's ever had.