From an old post of mine:
Vivec admits that he killed Nerevar at http://til.gamingsource.net/characters/trial_vivec.shtml
"As Vehk and Vehk I hereby answer, my right and my left, with black hands. Vehk the mortal did murder the Hortator. Vehk the God did not, and remains as written. And yet these two are the same being. And yet are not, save for one red moment. Know that with the Water-Face do I answer, and so cannot be made to lie."--Vivec
There are also messages in the http://til.gamingsource.net/mwbooks/lessons.shtml where Vivec says he did it.
"He was not born a god. His destiny did not lead him to this crime. He chose this path of his own free will. He stole the godhood and murdered the Hortator. Vivec wrote this."
Also note that the 36th Sermon which references the battle at Red Mountian has the message "FOUL MURDER" hidden quite plainly in it.
Further, when you speak with Vivec in-game about 'bitter recriminations' he says:
"You want to chide me... why did I murder Nerevar? why did I break my oath to Azura not to use Kagrenac's tools? why did I cause others to suffer?"
He then goes on to say in the topic that is brought up 'murder Nerevar':
"We did not murder Nerevar. The legend that we murdered Nerevar comes from a story told by a shield-companion to Nerevar, Alandro Sul, who lived among the Ashlanders. The Ashlanders have retained Alandro Sul's account as part of their oral histories. The account is persuasive in some details, implausible in others, and is in any case false. I have two accounts of Nerevar's death here in my library. Read them, and judge for yourself."
So, rather subtly, he says 'We didn't murder Nerevar, I did.' He doesn't lie to you, he just doesn't tell you the whole truth...