Oh, I've yet to lose anything quite so important as sanity.
[ In fact, he seems to have regained it save the irritation evident in those golden eyes that seem to flare ever so slightly at each syllable. But he's calming himself. No sense in losing it completely. Not yet. Not now. Not here. Not with Acumen's eyes everywhere and every step out of line meaning restraint. He'd been used to more subtle tactics after all, hadn't he?
He lets the other demon pass, lets the usual mockery slip past those lips he wanted nothing more to silence eternally. Oh, he'd have his moment. There's a moment of calm however just before the moment Sebastian passed right by his side.
Then he speaks. ]
I trust that with you here, Ciel Phantomhive is left to fend on his lonesome...? Imagine. That delectable soul completely unguarded.
[ A sneer. ]
And you call yourself a butler of higher caliber than most.