(NOTE: Many fans who've been with the series for a while might read the following response, and then justifiably suspect some kind of prank or trolling. So let me just reassure the suspicious reader that everything here has since been confirmed by Brandon Sanderson through his verified Reddit account in a public discussion here: https://www.reddit.com/r/WoT/comments/3snjii/spoilers_allsignings_and_secrets_terez_reveals/. :) )
The big murder mystery of Who Killed You Know Who is easily one of the most heated and persistent debate topics to come out of the series, rivaled only by the Taimandred conspiracy theories. Though both questions did eventually receive canonical answers by the conclusion of the series, it turns out there is somewhat shocking epilogue to this little story.
Sometime after the publication of Lord of Chaos and after RJ's remark about the killer's identity being "intuitively obvious", he later apparently made a decision to go back and change the identity to the individual revealed in ToM. Clearly this alteration renders the "intuitively obvious" null and void.
The first of many questions people might ask, is "Why is this only coming to light now?" Well, few months back, a passionate and well-known fan in the WoT community by the name of Terez, obtained obtained permission to read RJ's master compilation of personal notes the story, and eventually stumbled across a few curious statements. Terez wrote up detailed account of the findings in a post here, but tabloid headline version reads:
TAIMANDRED KILLED ASMODEAN.
At least, that was the case at least up to the publication of LoC.
In a file of notes made post-publication of Fires of Heaven in preparation for Lord of Chaos, it is written:
Demandred: Hated/feared/despised Lews Therin. Like Lanfear, he plays for larger stakes than most of the others, who are trying to stake out worldly kingdoms. HE WILL SHOW UP CLAIMING TO BE MAZRIM TAIM. TAKING ADVANTAGE OF RAND'S AMNESTY.
Again, later in the same file, a section of notes about Dumai's Wells contains the paragraph,
Taim/Demandred showed up, not so much because his party wants Rand free -- though that might be a point in their plans; on the other hand, Rand in the hands of the White Tower, and thus within Mesaana's power, could still cause one hell of a lot of chaos -- but because of learning that the Shaido were moving in. They could not be sure the Aes Sedai could drive off the Shaido, nor that the Shaido would not kill Rand. And a rescued Rand, pissed at the Aes Sedai will really be a source of chaos and disunity.
The other big revelation comes from a related file of notes pertaining to (of all things) Nynaeve's arc up to the end of LoC, where we find the innocuous comment,
She does not know that Aginor (Osan'gar) and Balthamel (Aran'gar) were resurrected, the latter as a woman who is now masquerading as Halima, Delana's secretary/companion.
She knows that Moghedien was prisoner, of course. Until she is/was informed by Egwene, Siuan or Leane, she thinks Moghedien is still a prisoner.
She does not know that Asmodean was a prisoner of Rand, nor, of course, that he was killed by Demandred.
So there you have it. The answer to your question is that the question itself wrong. Another implication is that, yes, RJ apparently outright lied to us on at least one occasion where categorically denied that Mazrim Taim is or ever was Demandred, and that may not sit well with some people. If that's the case, I refer you again to the reddit thread linked to at the top where Brandon Sanderson, as a passionate fan himself, offers his expert insight on both possible motivations for the change, as well as the far-reaching unintended consequences of the change (including an explanation for why Demandred's Shara arc in the final book felt so jerry-rigged and out of place).
"“Surprising what you can dig out of books if you read long enough, isn't it?" -Rand Al'Thor