Actually makes sense for once for a party to eliminate one of their own and nobility wanting to get rid of him too. This time, it's an entire guild of thieves that's on the line, the King promised his end of the deal, so he has enough reason to want him dead. And this also justifies the party backstabbing for once, since (while visibly enjoying what they are doing) they were ordered to do so, and while they did use him as bait per the flashbacks, it isn't like they ever took direct action to kill him until ordered so.