Motierre is a messenger and probably not even the real name. And if you looked carefully, there were two "Motierres" so as not to give you any way of blackmail. He playing his part. No one on the Elder Council is going to want to show themselves to be directly connected. As a matter of fact I would not be surprised if Motierre was a messenger who was given the task by someone two steps removed from the Elder Council member(s) involved in the conspiracy. His role was to 1) perform the dark ritual; 2) meet with you and give you the job; 3) the second one's job was to make good on the payment; 4) report back to his superiors that the DB did the job. The Elder Council ASSUMED that things were still done as in the old days. Fortunately for them the Night Mother arrived in the nick of time.
My hunch is that you initially met the real Motierre. The second contact for payment was a lackey.
Motierre fulfilled his task completely. There was no deception. The only deception was done by your predecessor, Astrid. Astrid said "So
someone is trying to rise above their station." She didn't say who. She assumed and she left that to you to assume, and we know what A$$ U ME means. Also I've concluded that Astrid is a bumbling idiot who was just winging it and making it up as she went along. No sense of honor. No sense of purpose. And she stabbed you in the back which to me discredits her a whole lot. Motierre will not become Emperor. By killing Motierre's messenger, you sent the Elder Council a message that the DB is not to be trusted with future business.
This was a conspiracy that went quite deep. Not all the eggs were in one basket. So my message to those who killed the messenger: "You're fired!"
PS: I would not be surprised if the conspiracy originated within the Legion. The Romans used to have the Praetorian Guard carry out these assassinations all the time for a general to take over.