Why is it that Sebastian seems to somehow be in control when dealing with Our Ciel, but appears to be dumbfounded when dealing with BD Real Ciel? Has something changed within this demon butler? Where is Sebastian's sneakiness, and underhanded cleverness?
The best I can tell is because Sebastian is bound to our earl in that contract, which has some wiggle room for him... and even though there's some novelty about this contract, it's not his first. He knows the loopholes in the contract and can act accordingly. And he knows what makes his young master tick.
But he's still trying to process exactly how real Ciel even functions as well as he does. He ate real Ciel's soul, but that doesn't seem to bond them together. Not like that, anyway. He's honestly confused.
He has no control over anyone who is soulless. I don't think he can comprehend their thought processes and might not be able to anticipate their moves or choices too well, because of that. Hell, he can't even tell a bizarre doll is in the same room, let alone anywhere within the vicinity, without actually seeing them. That's why he couldn't detect Arden and his friends at Weston when he used demon senses to search for them, felt weirded out when he touched Agares' hand, and couldn't locate any of the star lords at Sphere Music Hall -- they were probably simply hiding just out of sight.
This is a major learning experience for the demon. This is completely outside the purview of his knowledge. It's new to him... and kind of scary.
I should think it worries him also because he must now question what he's said in the past about not being able to get back what was lost.
Of course, what has returned isn't quite the real Ciel that our earl once knew and cherished. It's a nightmarish approximation. Merely a physical manifestation, literally without soul. But the physical manifestation still evokes deep emotions.
So, the demon finds himself in the weeds on this one. How do you handle a reanimated corpse that's "chock full of episodes"? Can you just cut it down in front of its loved ones? Our earl doesn't accept this bizarre doll of his brother as "his brother returned", but I don't think he would handle it well if Sebastian just went over and crushed real Ciel's head. He has to let his young master decide how to handle the situation.
I still anticipate their final confrontation to be more emotional than physical, between the twins. Maybe real Ciel will be more physically aggressive than our earl. And for Sebastian to be somehow barred from intervening on his young master's behalf... much like how Lucas didn't want to harm his dead and reanimated older mirror twin, Claus... and how Boney couldn't protect or fight on Lucas' behalf... during the twins' final confrontation in Mother3.













