I'm really interested in this thought but Ii'm confused like there are two ciel's?
YESSSSSSS, I completely agree with you! ヾ(❀╹◡╹)ﾉ~
Yes, the Ciel we know had a twin - both of them were kidnapped in the past, and the other twin died right before Sebastian appearing. This other twin was the real Ciel, and the Ciel we know decided to use his brother's name after he was murdered. The thing is: the other twin, the real Ciel, whose blood summoned Sebastian, is back and probably seeking for revenge or being manipulated.
A thought: when Ciel asked Sebastian about lying to him, I think Ciel is referring to their contract. It's been a while since I read this bit, but I think it was a human sacrifice that summoned Sebastian. If we go with the twin theory, then it was the real Ciel who supposedly died on that table. I believe the message on the wall is something only he and his brother understand, and if that's the case then it would imply the brother is still alive. This is why Ciel questions Sebastian and why Ciel looks really upset when Sebastian tells him that he didn't lie (about the real Ciel being dead).
If all this is true, then the real Ciel was brought back from the dead, most likely by the Undertaker. That's my theory anyway.