Yeah....I don't think killing the Oracle would be a good plan...
At this rate, is sounds to me like you're trying to resurrect the Dread Army or something and have another siege on Serendipity. If you're plotting with nobles, chances are they're not going to want to destroy the entire system just because they can...Most nobles would not risk the security of their station and the kingdom for total anarchy O_o Especially if they come from a long-prestigious line of nobility! I seriously doubt that anyone in their right mind would want that type of chaos....
If you wanted to go and destroy the system all together, I would recommend shying away from nobles, because it's less political intrigue and more all out war....if you went to attack the Oracle or the heads...you would be making a butt load of enemies! >_< and I bet most of the commoners and even the nobles would rise up against you and beat you to a pulp....
Remember to try and keep it a little bit realistic ^^;;;;;;;