It's apparent the way that Terezi bristles at his words. Briefly, she considers slamming the door in his face. But that won't solve anything, even if it might make her feel better.

"Karkat." Her tone is even, measured, neutral. "I'm not sorry for what I did. I'm sorry for making a promise that I couldn't keep, but that's it. That's the only apology you'll get out of me. I'm not sorry for killing him. I'm even less sorry now for disliking him as your moirail. You don't deserve anything from me for that, after the way you've been acting."

And on that note, some of her anger fades. She hesitates, uncertain, then continues. "...I don't like what he's done to you. You were never like this before. Even if you were a pain a lot of the time, you never would have asked your friends to die for someone else."