Like a shadow of her, the more emotional Lily gets, the more it drains away from Severus. He sits across from her, still and blank.

Oh. He promised her. Well then. That's understandable, isn't it. James Potter, paragon of truth and justice, promised something, thus making it utterly inconceivable that he might ever go against it. The poster child for the debate of 'designated dark magic vs intent making good magic dark' promised he would stop doing something that he and his best mate were obsessed with, and Lily, brilliant witch, prefect, head girl, believed him.

Severus doesn't say anything. He stays there at his desk with his gaze cast to one side of her, silent. His options here are telling her it's fine - it's not fine - or going on about the things her husband has done, which she should have known he'd do, because James Potter is James Potter and Lily isn't a bleeding idiot. But maybe she had nothing else; maybe Severus was so awful that going to Potter and duping herself into believing that lie was easier than dealing with Severus's painful truth.

He thinks it was better before this conversation, probably.