Keltham will read these more understated reactions as "probably not faking it to herself hopefully??" and "demon fighters have high pain tolerance go figure", combined with not being all that sure of how much ripping out armpit hairs is supposed to hurt in the first place. He wants to ask Carissa exactly where it stands on a dolor scale, or order her to say what she's feeling, thinking, during this, but it doesn't feel like it fits... well, it fits him, it fits dath ilan, but it doesn't fit the image she constructed for how to do this new thing called Evil.
The way that she's holding her arm in place, though, keeping herself exposed to the pain, like she's reaffirming the gift of herself and the obedience, that part is working for him. By the time he finishes making this armpit symmetrical with the other one, he thinks, he's going to be more than ready to - well, the notion of "use her" or "take from her" does seem right, even though the notion of her as an already-bought good or service doesn't.
(There are so many things he wants to check if he's doing right. But Keltham is succeeding in telling his mind to stand down about generating infinitely many important questions while having sex. Keltham is not the first dath ilani ever to have encountered that issue, and Civilization has turned any attention to making sure the species doesn't die out because of it. He can just use the standard trick of promising himself that he'll write up an after-action report and submit it to Carissa for review later while they're not having sex.)