"If you must know." Their smile is the perfect, plastic thing it always is, rigid and unremitting, a perfectly sunny endeavor.

"Mother dearest, you see, has a fondness for employing violence to keep the children in her care safe." She sees it as acceptable, of course. Prove to me you are strong enough to survive. It is for the best, because she knows best for you, of course. "She runs the risk of killing them, but when has that ever mattered? Why should a child be concerned, if those who claim to love them hurt them for their own good?"

It is a familiar tale, is it not?

It is doubtful, they think, that he will believe them without fail. But they cannot say that any of it, not a single word they have spoken, is a lie. They have certainly lived it often enough. Felt Frisk's skin crisping as it burned off their frame and they screamed, cried, pleaded for Toriel to stop.
Saw her face when one too many little balls of flame struck home.
But Frisk, of course, cares little for that.

Frisk has always chosen to embrace the things that hurt them most.