But I seem to remember that Sebastian wasn’t originally hit like that:

He clearly “died” on his front, not his back. But then…

…when it was time to resume the dying pose…

…Sebastian CHOSE to lie on his back.

He even chose a specific look for his “death.” And before you think this was solely for Earl Grey:

Sebastian says he “meant for it (the changed pose) to be a little revenge, BUT”- implying that the “revenge” he intends isn’t on Earl Grey.

So who was he getting revenge on? My bet is our other main character, Not-Ciel. I think the reason why Sebatian’s death pose looks similar to Vincent’s death pose is because Sebastian copied the look Vincent had upon his death. This would’ve been partially to tease Not-Ciel, and partially to ensure not-Ciel’s acting was 100% convincing; if the fake trauma mimics the real-life trauma from Not-Ciel’s past, then there’s a chance of it invoking some of that old trauma and provoking a genuine distraught emotional reaction.

Which would justify Ciel’s “unexpected” revenge:

While there’s still the mystery of why and how Sebastian knew what dead Vincent looked like, I don’t think this means Vincent is alive, or Vincent is Sebastian.

But then again, I could be wrong.

Or Ciel may have had a twin but this guy isn’t him. It’s a clone. 

I’m sorry this is gonna be rambling but I’m pasting in my theory before I lose the thread of it again. 

These is evidence Ciel had a twin or at least an older brother. In those days the idea of “An Heir and a Spare” one child for the family line and one to take his place should tragedy or Dishonor ensue wasn’t exactly uncommon. In royalty it was common and the Phantomhives are Bastard Nobles. Claudia Phantomhive doesn’t seem to have been married, her husband has never been mentioned, and she was important enough to Undertaker for him to keep a momento mori of her.  In royal and noble families if they had multiple children, The First was the Heir to the fortune, the second took the Business, and third would take either Religious Studies or Schooling for something else. If both survived they’d have likely worked together with the eldest as The Queen’s Watchdog and his brother as The Head of Funtom Corporation. All that aside, this isn't his twin.

Undertaker said himself that, once lost, a soul can never come back.This is a clone.

It would make sense for why he took blood and looked at Sebastian’s memories. He needed a frame of reference. 

My Theory based on what the 4 stars said and the fact one of them (I think Polaris) said that 

“It’s only a matter of time before the Blue Star Falls. And should that happen, my pride as a butler would not allow me to die without action.” 

Which is a really SEBASTIAN thing to say. Comparably though, their room is sparse and very much destroyed, almost as if they’re prone to fits of anger? Or perhaps they dislike being caged so. 

Aside from that, and I’m omitting some details to avoid spoilers, but there’s only two people who can move fast enough to surpass Agni in a fight. Sebastian (with difficulty) and Elizabeth who is a close second to Sebastian’s speed who could likely match Agni. 

I often jump to conclusions about what will happen in Kuroshitsuji but shit son I think I might be onto something.

So what if Ciel had a brother? Close in age, maybe a twin? Either way he would look like him. Here's my reasoning:

I've been thinking about in chapter 90 when Ciel was saying that 'at least he had him' - some mysterious person around his age who was dragged out of the cage and presumably killed.

Next I noticed that in Toboso's art book there is a portrait of Ciel as a child holding hands with someone who has been burnt out of the frame. Then, rereading the Circus arc today and there are flashbacks from when Ciel was kidnapped of who we presume to be Ciel forced out of the cage and onto the pedestal, and one image even shows him dead.

Then I went to the Campania arc. I feel like I was pulling slightly at straws for this one but notice the graveyard. Most of the graves are understated but the Phantomhive graves are slightly more built up than the rest of them, identical only to each other and in a perfect row. There are four of them, one for Ciel, Vincent, Rachel and one other.

And finally, and before I found this one I wasn't going to say anything, but in Lizzy's flashback during the Campania arc there is a panel where Ciel is being scolded by Francis and at the edge of the panel Lizzy is sitting at a table with another child their age, who's upper body is covered by speech bubbles. It occurred to me that it could be Edward but the legs are too small, he'd be older than that I'm sure.