What reason would Lara have to assassinate The Merlin or other Senior Council Member? Except for the knowledge that Luccio may have given him, about Thomas mother, but the Merlin may already be using this knowledge to get rid of Harry, so Thomas was intending to assassinate the Merlin, but the Merlin was already trying to set Thomas up to look as a potential assassin and bait for Harry.
What reason? I guess that is what we are about to find out in Peace Talks. For that matter what reason would Thomas have? Only one I can think of is he made a Faustian bargain with someone to keep Justine and their child safe from the Hunger. It could be he made that bargain with Mab, or Lara made that bargain on his behalf. Then the question becomes, who does Mab want dead? And
it would fit with her four dimensional game of chess as well. She wants someone on the Council dead, she may even have a good reason to want him or her dead.. However to keep a level of "plausible deniability" she cannot order Harry to do it. And further if it was a Council member Harry would fight her on it.. So she makes a deal with Thomas, enter Lara, who will want to save her brother so, more favors asked for and granted, and Harry finds himself a pawn in a dirty game to do what he would have tried anyway, save his brother.
Actually if you go by what is taking place in the first six chapters, someone wants this member dead, and Harry is being set up to take the fall, which Mab is trying to prevent.