In their perspective, (this faction of) the White Court's philosophy requires waaaaaaaaay fewer mental gymnastics to justify than, as the spymaster would put it, “the sanctimonious contortions of a death cult, piously pressing poisoned chalices into the trembling hands of their pitiable flock.”
"We were created to shepherd the kine, to humbly shoulder a fragment of Christ’s burden. Where we hold sway, we consume the fraction of humanity that foments chaos and unrest, feeding upon the emotions and individuals that sow discord. Their cities flourish under our hand. Without us, the inmates would run the madhouse.”
They can make the Steven Pinker style arguments demonstrating prosperity. In fact I just realized that this concept is basically just Evil!Steven Pinker. Maybe your spymaster was a mentor of his.
To which William could potentially respond that they gain control, he blinks, and within a decade they're selectively shaping the "pitiful flock" into slaughterhouse-tolerant obedience while/by engineering society to become totally enslaved to lust, rage, fear, and despair. Intuitively the first place I go with that is a stagnant culture that stops growing and just goes through the motions of existing. The population has no life or will left to do anything but writhe underneath the perversely disciplined facade required to maintain their good relationship with their fixers. At least until a culture that didn't do that comes along and takes over, anyhow (perhaps backed by a Knight).
You could probably explore a lot more subtle flavors of (not quite) eternal decay and degradation without the mercy of mortality/death, too. The painful mutablility of growth/death vs an ego driven neurosis for eternal stability is a perennial human conflict. It'll fit just about anywhere if you squint hard enough
Our spymaster would say that the same as the argument for peace (or "peace," if you prefer), it's also easier to make an argument against conflict. Behold, a common privateer causing mass suffering and the deaths of countless innocents. It has to end eventually, someone has to come out top. Better them than a seething cauldron of chaos infecting its way from port to port. It's not like the nobility of the White Court lack food or influence in such conditions. They're just not complete psychopaths "driven by the mystical urge to burn the sacrée campagne to ash. To feed the spawn of a Dragon no less." That the masses could have been so graceless as to confuse them for villains in the face of such opposition is a grave, yet characteristically vulgar sin.
Getting all of those stories about white knights killing dragons going was barely propaganda, according to their records. That success is what gave him the idea for Stoker and the Black Court (and it was his idea, no matter what that Americanized "daddy's girl" Lara Raith says). The Whamps of 1333 simply had to associate the color of their fair Court with a few cherry picked stories about what the Knights of the Cross were doing anyhow. Then they had their boy Edward III make a noble title out of the cultural concept, gave it to the Irish to spite the Welsh, and therefore fed their rivalry to keep them both busy and prevent them from contesting the Whamps' inevitable political victory for another 70 years. Until Owain Glyndŵr and Y Ddraig Aur got involved, anyhow. Adapting the concept to novel fiction was simply a necessity so they could freely leverage such maneuvers of renown without polluting the political titles of their mortal assets.
Besides, does not taking up cause against God's enemies, the Dragons and Swords notoriously being at frequent odds, put one on the side of righteousness? Honestly, losing the identity of Wales to a manifestation of madness itself, even after they had managed to seal it under a castle for a time, reveals a deep sickness crawling forth from the heart of that isle.
And then William calls bullshit, puts him in a headlock, and makes him repeat the last things he said in a victim's bedroom under magical oath. Or however you'd like to characterize him pulling the rug out from under typical White Court diplomatic shenanigans.
You could go back and forth forever, it's a pretty robust duality. Mechanically that could translate into a pair of aspects, maybe even their Troubles, that constantly provide justification for the GM to invoke when they're in the same scene. If the GM prefers a super liquid fate point economy, you could each get as much as a compel every round of combat as their fighting styles conflict. Rather than tossing out mind bullets from behind cover, the whampire spymaster might be compelled to create pockets of stability and control (if not necessarily calmness) so that he can actually influence anything as a primarily social/mental combatant. Then the mystical force giving William his powers subtly influences him to disrupt it, since that's basically why this metaphysical category of energy exists. Not enough influence to overwrite his free will and waste his turn, but he might get tunnel vision on smashing that particular pocket of resistance (and the horrible stillness spreading from them) instead of his actual most rational objective.
If you set it up that way beforehand, you can turn each conflict into a fate point printing session, and as a bonus each encounter become super dynamic as absolutely nothing ever goes to plan. The extra fate points should be enough to punch a refresh or two above their weight class, and with some practice you can even work the compels into a kind of teamwork. When one party member starts getting overwhelmed and has to throw around a ton of power, the other is automatically drawn over to where they're most required. To help. Drawn over to help, they both meant. Totally.
As for why they would ever work with each other...
For William, an optional Faerie tie-in could be an absence of both Summer *and* Winter in an area was bad for their business, so Mab and Titania mutually hired out a Dragon to take care of it as a neutral third party (whatever shenanigans it's using to metabolize both Summer and Winter power without exploding could be an interesting plot hook, and would explain why William isnt himself attached to Faerie). Managing continent-to-planitary scale forces of nature being their day jobs, and Dragons are one of the few entities both Queens know that the other can't bully into favoritism. Downside, they're expensive as hell, so its gotta be a Dragon-scale problem, like the mysterious waning of the influence of nature itself.
After discovering that these dead zones are a Mortal Thing in origin, and therefore beneath a Dragon to handle directly, said Dragon (maybe Y Ddraig Aur?) will put a half mortal scion, Y Ddraig Goch, on the job. Who then subcontracted the job out *again,* to a Mortal(ish) Knight. Ultimately the source of the problem was discovered, and the problem was solved partially by the Brothers Grimm planting the Fae into wide mortal tradition, and ultimately by Walt Disney tempering the Fae directly into the mortal yearning for a hollow, what we would now call plastic, existence.
Yes, Mab did alter the very nature of all Fae to preemptively prevent another Iron style allergy. Have we mentioned that Mab is fucking scary. Downside is that she actually hates the results, and hates Disney for them. Honestly she got off kind of light for doing the immortal version of violating the 2nd Law, and she still pawned off the weakness for mortal "Peace"/stagnacy to the Dragon line feeding off her (and Titania's) power. So they're keeping the vassals around and finding work that they both can agree on. They don't agree on much.
Even as scapegoats for a weakness, it's still a basically free lunch, and I don't think Dragons are metaphysically capable of saying no to offered wealth.
But now Mab and Titania actually have a specific request, and they put it in with his god's god (or his dad's dad?) as that guy's Patron. And it's a request to tell the mortal to accompany some other mortal on some bullshit mortal errand. Y Ddraig Goch will tell him that he's going along with whoever the Queens damn well tells him to go along with, and he'll be ffwcio pleased about it (Take a Compel).
The spymaster, for his part, has been given his orders face-to-face from the head of his house. And not even in that wink wink, nudge nudge way. He actually has to help the mad Knight, or they'll kill him. He even asked directly, like a gawping peasant, to be sure. Le petit seigneur confirmed it. If he is to watch a ship sink, he'll muse with an air of poisoned magnanimity, it’s only proper to help nail the planks together first. He'll do exactly as ordered, and not lift so much as a finger to help when this lunacy reaches its inevitable conclusion.
Which is pretty much exactly the reaction the Archive had calculated from him, and one if the reasons why she gave the order to to tap him in particular for this project. His boss being one of the White Court involved with researching how to kill supernatural sources of power in the Oblivion War, that being the objective their projects over the centuries have been focusing on. This fact is also why Mab is just eating the cost of buying off a weakness, instead making the whamps the first Court to bite it for the audacity.
Getting the Archive of the era to sign on to publishing the Big Book of Vampires was a headache, but promising to shift the mortal conception to "sparkles and smoldering angst" was enough to sway her. Because then the demons inhabiting the corpses can't use it as a tether. Yes, really. YES, REALLY.
Why the Archive and Queens are coordinating to send these two into a series of obviously cursed holes in the ground could tie into the larger plot. Why they specifically chose these two can come down to having an ideal mix of being quasi-local to the plot (situationally if not necessarily geographically), their antagonism towards each other to maximizing the free will they can express from under their templates/mantles/conditions/whatever (at the cost of their personal sanity), all while still having the prerequisite phenomenal cosmic blow-shit-ups to actually get the job done. Also they were already paying for these guys anyhow.