That reply is...unexpected, to say the least. Rarity does not like to be caught staring, and yet that simply keeps happening in this conversation.

And then Sans continues, and she feels her cheeks redden with shame. He's right - she just lectured him on hurting people, when she made a terrible decision and hurt him right back. "No," she finally replies, forcing herself to keep eye contact. She can't look away for this. "I don't. After everything that's happened - after all of this-" She shakes her head, takes a deep breath, and tries again. "I made a poor decision. I never should have hit you, and I'm sorry I did." It's more vulnerable than she'd like to be, especially given how many times today he's thrown her words back in her face, but she owes him this.