Frisk stays where they are, sharply pulling their arms against their chest and holding themselves angrily.

"Didn't need to make me think you were a stranger. That there was another lab baby. Didn't need to lie right when i talked to you. Didn't need to disappear. Didn't need to make me think you left."

Ah. And there's the crux of the matter.

There's very little they can think of that would be worse than Sans disappearing.