So, you have the Ascians: Their plan is to raise up Zodiark, whose power will let them seed the star with new life, and then kill all that life to bring back their fellows. Listen to Emet-Selch talk about the Sundered. They're lesser beings, they're unworthy, their reduced aetheric density and inability to create makes them sickening reflections of what used to be. The Ascians have power, so they can enforce their will. The Sundered don't have power, so they don't deserve to stand toe to toe with the real people who actually matter. Emet-Selch's grandson was a disappointment because he didn't have the same power as an Ancient. The Warrior of Light is a disappointment because they don't have the same power as an Ancient.
And this ideology of power being everything is reflected in what the Ascians create. It's not an accident that the Allagan Empire expands and crushes their enemies with an iron fist, doing massive injustice and inflicting untold suffering on all sorts of people in the pursuit of greater strength and control. Political power, science, magic, capital, creation-- any form of power they can get their hands on, at any cost. They're literally destroyed when a fancy power plant is too strong for them. You see what's happening here, right?
When the tribes are being pushed back by the Eorzeans, the Ascians show up to offer them rituals to summon Primals. By destroying life and causing harm to the world around them, they can become stronger and in that strength -- with their will, conscience, and humanity suppressed in service to something greater -- strike back against their enemies. The problem is, it's not just destructive to their foes, but corrosive to themselves. It's those same themes echoing again. They echo down sidequests, they echo down other forks of the MSQ, in large and small ways. Nanamo, Raubahn, and the Monetarists. The Ivy. Hien's questions about reclaiming the throne. Yotsuyu's whole arc. Eulmore's tower and the rickety town below. The Weapon arc, and Bozja too. FFXIV is regularly examining what power is, who has it, what kind, what they use it for, and what the effects of it are.
The Garlean Empire is set up by Emet-Selch as a successor to the Allagans, in the same mold. It has that same ideology as the Ascians, on a smaller scale: that people who aren't the right people are somehow lesser for it. That strength is what matters. The only way a lesser person can earn real distinction and citizenship in the Empire is to literally give their strength at arms to the cause. That cause is itself to trample all over the supposed lesser beings. It is to seize what they want at the point of the (gun)sword. The Empire conquers and expands, it puts its boot on countless necks, it gets deep into unethical science just like the Allagans, and nobody has the right to say anything about it because nobody can muster enough force to do so.
It's a harmful way to live, both to the people who encounter it and to the people who perpetuate it. The plan is for it to eventually generate a Calamity that's going to be equally destructive to people in the Empire, whether that's through its own hubris or inspiring uprisings against it or some combination thereof. It's very successful at this: not only does it actually work once, it does a second time in an alternate future, and it had at least one more good shot at it foiled by the WoL.
But that ideology isn't just a government-level thing, right? It affects the people who are living under it too. Look at Varis: while Emet-Selch cared for theater, he bans everything that doesn't meet the standard of the Imperial censors because it's not something that generates strength. The ideology is reinforcing itself, becoming more concentrated. And then he finds out that Garleans are actually not the people who matter, like he's been told his whole life! Instant inferority complex. The only solution he can think of for this is to accumulate more power by becoming one of the worthy few-- speedrun killing untold numbers of people to return everyone to Unsundered status and then fight the Ascians themselves!
Look at Zenos, too: Here is the Ascian and Garlean might-makes-right ideology crystallized into one person. Strength is all that matters, so he becomes strong. Getting what you want, without regard for the cost to other people, is what's done, so he gets really good at that. Grasping for more strength, using it to crush others in the hopes that one of them might eventually inspire an uprising against him is literally the whole thing he's doing in Ala Mhigo. Zenos is power as conceived by the Garleans and Ascians, not held in thrall to conscience or pity or kindness, used only to get what you want and forget the consequences. He is everything about the way those powers operate and think distilled into its purest form.
If power is the only thing that matters, running into someone more powerful than you is a calamity. Varis gets in the way of what Zenos wants, so he cuts him down. Garlemald is a smoking ruin, because why would Zenos care about that? They're weak and not worthy of his attention. Elidibus' plans and schemes revolve around the Empire and Zenos' body, but the instant they become inconvenient to Zenos, he throws them into chaos and sends Elidibus running for the hills. They have made the perfect monster in their image, and it turns out that when you take that ball and run with it, it makes a monster that terrifies even them!
The only thing that brings Zenos any joy is the WoL. The one person with enough strength to face off with him, an actual equal, someone he can finally think of as real, in the same way that Emet-Selch was hoping he could feel about the WoL. And the only response he has to it is the thing he's been taught, the ideology he's been surrounded with since he was born. He accumulates power and makes plans to accumulate more power, including planning on cracking Zodiark open like an egg and sucking out all the gooey crystal juice. If the world burning is a necessary piece of that, hey, fine: he can make that happen, and what are you going to do about it? Because if you're not strong enough to keep up, you don't get a seat at the table.
Zenos is everything you've come up against in the story coming back, echoing, repeating itself for emphasis. It's like poetry, it rhymes. He's the thesis statement, the central thing the story has been circling around, discussing, thinking about, but magnified and centralized. Confronting the question of Zenos is a wonderful way to put a bow on the story of the Empire, the Ascians, and the Unsundered all in one go.
It's tied in closely to another question for the reader to consider: hey, the WoL is strong too, right? What's the difference? Is it that they're tempered by conscience? That they have friendship and companionship? Their strength is used for service: to defend, rather than to seize and destroy; is that it? Is it just circumstance, would there be no difference if things shook out differently? Maybe it's just because power-hungry monsters drop better loot than dirt-farming peasantry on average!
In the end, the writers' answers and the readers' answers will probably diverge based on your experiences and your viewpoints. But I can't imagine Zenos, and the contrast between him and the WoL, not being an important part of shading both that question and answer. That is an opportunity that is unique to the character he is, in the position he's in, from the history he has, that I feel like can't be effectively replicated by just bringing in some new guy.