It wasn't that she didn't understand, because god she did. There were times, even after her possession by Peter that Lydia couldn't tell what was real and what she had dreamed, and she knew how terrifying that could be. There were moments that she couldn't recall what she had been doing even hours before that would send her into a panic. Lydia knew what it was like to lose her mind. It had already happened to her. "I don't think that you're evil," she confessed softly. "But I don't know what else there is I can do to make you see that you aren't."
Because of course in the end what mattered to him was her staying nsafe. That's how better people than she thought. Lydia was selfish; she doubted she'd be able to give him up even if keeping him around meant that she would lose any chance she may have had at a future.
Her heart was aching for him and Lydia wanted to take away all the pain and strife that had made him susceptible to the nogitsune in the first place. If there was a way, she knew she would take this burden from him without a second thought. She would do most anything to keep her friends safe, and even more to keep Stiles safe. Her fingers tangled in his hair, playing gently with it and scratching lightly at his scalp.
Lydia didn't want what he said to make sense to her; she wanted to be ignorant for the night and not let the reality sink in, but she knew he was right. There was a chance that he could seriously hurt her even if he didn't want to. "You're not going to lose me. Not to this. I'm going to be right here." She leaned forward until she was at eye level with him before kissing him gently, her tears mingling with his own. "I'm your anchor. We'll figure it out."