[Oh no, how could he? Deliberately mislead and keep secrets from his own master-- whatever came over him? He feels so guilty now. No, really.]
I cannot think of what you are imagining to be a worse thing to obscure than this. [This whole "being" human nonsense, that is.]
Which you now already know. ["What more could I possibly be hiding from you at this point?" Yes, Sebastian, what a confidence-inspiring way of not actually responding to Ciel's actual concern.]