If you don't want the motivation to be for the king because they liked the king, you could perhaps try giving them a weakness or something that someone else in the group could exploit to get your character working with them? Or maybe just set it up so that their history gives them a reason to dislike Bernard of Ghant or one of the other monarchs enough to use this chance to try offing them, and the group of angry courtiers(and others) might make a good meatshield or alliance? Maybe?