It's the answer to the question Sirius had been leading to and Sirius is only surprised by Kell's honesty, though he's beginning to think he shouldn't be. However macabre the man's powers may be, he doesn't at all seem devious or calculating.

Then again, Sirius isn't always the best judge of character.

"Tell me, does he care for you as deeply as you do him?" Sirius asks, reaching once more for a chip, though he only holds it, eying Kell intently. "Does he give even half as much of himself?"