I lean towards Nightwalkers, and I don't see how "Tearing down the establishment" is no longer an option. I if remember it correctly when first discussed the reason for the tear down was that our guys really hate the way things are going, with the baby slaying and mass genocide of innocents. I think starting with that as a base would be a good way to worm our way in and find the true course of the plot and eventually work out the spectacular story I know is hiding in here :P I know it sounds a little "Good guy" but one does not need to be good to disapprove of killing people not really involved or babies. Think of it like the organization, while still keeping war at bay, are doing so in such a corrupt and self destructive way that now it's no longer about keeping the peace, but about the ones in power asserting their authority in reprehensible manners. That would be the base motivation of the coup, but since you wanna avoid "Good for the sake of good" your guy could have his own reason for wanting to do what they are doing, and hey, maybe even be planning to screw my guy over once the opportinity presents itself :P