It was a slow build to understanding, but as soon as she got to the door Finn realized his mistake - he hadn't been wrong about how she would feel, just about what she would do with those feelings. The cold and calculated of it made his stomach ice over, and at the same time anger started to simmer. He knew she'd been through a lot, logically knew she couldn't be expected to be rational right now but at the same time, he couldn't stomach being used this way, not by her.
She was being entirely unfair, and even aside from that, she was making Finn feel like an idiot and there was nothing that amped him up to anger faster than that. "Are you serious right now?" he demanded, addressing her back. "You're just going to walk away."