I really love the new arc in Kuroshitsuji, I think it's setting up to be the best arc yet (even though we're only two chapters in). I really liked the Circus arc because I loved the characters in that arc I'll miss you, Joker, Beast and Doll but this arc seems to have the better plot, because I love mysteries.
Below the cut are my theories, some of which were already posted in the Kuroshitsuji LJ comm.
First of all, I think Ciel knew that Siemens was going to be murdered that night. Back in Chapter 38, he says to contact Undertaker. Now we know that Undertaker wasn't a guest at the party since he wasn't there. So that means Ciel was either after information or there was soon going to be a body that needed removing. There's also the fact that Ciel was quick to leave the party that night, which is suspicious. We know he was using his age as an excuse because Ciel is not a normal kid. So I think Siemens death is being used to draw out a more dangerous enemy, someone who hated Siemens so much that he wouldn't pass up the opportunity to kill him.
The other person who I think knew about the planned death is Grey. We know that Grey can take on strong opponents because we see that happen in the previous chapter, so he should have been a good bodyguard for Siemens. Plus he had a really guilty look on his face when the body was found, so I think he knew and felt guilty that someone left in his charge had to die.
I'm not convinced that Sebastian knew. His expression when the body is found indicates that he didn't know. I think Ciel is keeping him in the dark on purpose. Ciel and Sebastian likes to play "games" with each other. In the Prince Soma arc, Ciel entered Sebastian in the cooking competition hoping that he'd lose and in the Circus arc, Sebastian released a bunch of poisonous snakes in the area where Ciel was hiding. Keeping Sebastian in the dark now could be payback since Sebastain "dies" in this arc.
Moving on, I suspect the attack on Siemens happened when the bell rang for his room. I think it was ringing as a cry for help. We know he was still alive when Sebastian and Maylene arrived at the door because there is screaming coming from his room. So what I think happened is that the bell rang during the attack, the killer escaped while Siemens was still alive but dying, Siemens lets out one last scream and dies in the chair. This means is those who heard the bell rang couldn't be the killer (which is Sebastian and the other servants).
That leaves us with the other suspects.
Carl Woodley - Judging from the comments on the LJ comm, Woodley seems to be the person that most people are picking as the killer. They have a point, he looks evil and in Chapter 38 there was a newspaper headline about a man dying in the diamond business and Woodley is also in the same business. The guy is definitely shady and that's why I don't think it's him, he's too obvious. There's a reason why in mysteries it's not normally the most obvious person, because if the mystery is that easy to be solved it's no longer a mystery but is instead a triller. So I don't think it's him.
Lau and Ran Mao - I definitely don't think it's them. Yes, they kill when they need too and I think Lau knows more about Ciel and Sebastian than he lets on, but if they wanted Siemens dead they would have found a better opportunity than Ciel's own home.
Arthur - No, he's suppose to be the main character of this arc and his a real-life person. It makes no sense to make him the killer. Plus in Chapter 38 there's a voice-over from him talking about what happened that night.
Irene Diaz - If it wasn't for the Jack the Ripper arc she would be near the top of my suspects list. She has the perfect cover as a sweet and innocent opera singer. But we've already had a 'crazy female murder' and I don't think it would work with her as the killer since she'd just get compared to Madame Red. It's still possible that it is her (I wouldn't be that surprised) but I don't think so.
Grimsby Keane - Grimsby is I think the biggest wild card of the bunch. It's hard getting a reading on him since he acts nice at times but then it's also shown that he has a temper. I don't think it's him though unless something changes within the next few chapters.
Patrick Phelps - My current pick as the killer. Why? He's way too innocent. People that innocent always have something to hide. Lets look at the movie Clue, in the ending* the cowedly Mr Green is revealed to be an undercover agent, it's also revealed that the good-natured butler is the villainous Mr. Boddy. There's also the fact that Lau clearly doesn't like him. We know in the past that Lau only opens his eyes when he's being serious and he made sure to open them and look at Phelps. So yeah, Phelps is my pick.
We know from the foreshadowing in Chapter 38 that Sebastian "dies" and it wouldn't surprise me if there are other deaths in this arc as well. It's possible that the killer is going to try and fake his death to cover his tracks.
Regardless of what happens, there can only be fun times ahead for fans as we watch everything be revealed. xD
Yes, I know there are multiple endings but that one is suppose to be the real ending.