See while in some ways Nick isn't a bad candidate, (because he probably has the resources and knowledge to hide Harry even from someone like Eb), I feel like it doesn't fit because if Nick had gone after Malcolm, he'd probably have swooped in immediately and dropped a Coin on him.
An ignorant child Wizard would be a prime candidate for Nick to mold into one of his chief enforcers.
That makes a great deal of sense. Plus, do you think anyone who Nicodemus killed died with a smile on their face? Neither do I.
So yeah, it has to be someone that's capable of a subtle and near-untraceable Magical Assassination, and the resources and skills to conceal Harry absolutely from two major-league Wizards, both trying their Darndest to find him, but not enough to prevent Lea from tracking him.
I think there are three characters that we know who could have killed Malcolm Dresden in a subtle and nearly untraceable manner. Lara Raith is one, and the other two are Ebenezer McCoy and the Leanansidhe.
Ebenezer has a license to kill from the White Council; and from how we saw him behave in Peace Talks, if Eb thought it was necessary to kill Malcolm in order to protect Harry, I have no doubt he would have done so. However, at this point I have no specific evidence that he did so. All I know with certainty is Ebenezer described Malcolm as, "A mortal, without powers, without influence, without resources. But a man with a good soul, like few I have ever seen." So that suggests that Ebenezer Soulgazed Malcolm, maybe. That first sentence though, it describes someone who couldn't protect Harry. It feeds my suspicion of Eb as a suspect, but it's hardly proof.
We've only seen the Leanansidhe kill by using extreme magical violence against some of the Lords of Outer Night, but I don't doubt that she could do subtle if she wanted to. I think Lea might have wanted, or felt she needed to do so. People have wondered how Harry was disappeared. We know, or at least strongly suspect, that Lea was the women who occasionally visited Harry when he was in the system. We know she agreed to a deal with Margaret LeFay to protect Harry. I doubt that part of her deal was to look out for Malcolm. Jim has stated that Margaret didn't make the best deal she could have. As a personal note, Lea has always been my Number 1 suspect. If Lea believed that the best way to keep her deal with Margaret was to off Malcolm, or look the other way while someone else killed Malcolm, I'm pretty sure she would have been OK with either of those two things. Lea doesn't have a strong grasp of human morality, other than it exists and is often contradictory. I won't be surprised if we eventually learn that Lea had a side deal going with Justine DuMorne. It would have been quite convenient for Lea to allow Justin to train Harry. All she had to do was wait for Justin's inevitable betrayal of Harry so she could sweep in and rescue Harry so she could bind him to her own purpose. If you think about the conversation Lea had with Harry's disembodied spirit in Ghost Story, she knew an awful lot about Justin; specifically about his rotten character. It seems to me that Lea must have had personal dealings with Justin to have formed such a strong opinion of him.
Lara Raith, hmmm. Well, I suppose Harry might have been told an aneurism killed Malcolm, even if no actual cause of death was found. Lord Raith could have sent Lara after Malcolm, but wouldn't the more logical target have been Margaret's Starborn son? Wouldn't Lord Raith have figured that out? Killing a vanilla mortal doesn't make much sense. Also; if Harry was the ultimate target, sending a princess of the succubae to kill an infant is overkill in the extreme. It works to get Malcolm out of the way, but if Lara did the deed, this would mean that someone else already took Harry and hid him; and in that case why waste time killing Malcolm Dresden? There could be some missing piece that makes this scenario work, but I don't see it.
For now I'm leaning towards Lea as the guilty, or at least complicit, party with Ebenezer as my backup suspect. Lara Raith is a distant third. Then again, maybe they were all in it together!
I need to work out a scenario were all of our suspects; perhaps including Justin DuMorne, were involved in Malcolm Dresden's murder. Something like the movie Blood Simple (great movie, if you haven't seen it), where four different characters do or say things that the other characters aren't aware of or misinterpret, that eventually leads to three killings by the end of the movie.