And she's being unreasonable because he made a declaration in grief and anger, where a lot of people could hear it, because he just lost two children to humans. He reacted realistically, and then couldn't take it back because he couldn't dash the hopes of monsters who just wanted to see the sun again, who knew there was no way they could keep going like this. The game repeatedly brings up overcrowding. There's only so much they can do about that. They're running out of room and out of time. Resources are finite.

And all Toriel can think about is her own goddamn moral superiority.