This is an aspect of the Marley arc that I really loved. Throughout the arc there was a constant opposition between fighting to defeat the enemy or for some grand goal and fighting for the sake of protecting what you already have. I first noticed it with Reiner, his aspirations and the mentality of constantly moving forward with a single minded focus, land him in a depressed and suicidal state upon finally fulfilling it. He moved forward expecting to find a "hope beyond the hell" and what awaited him was yet another hell.
Eren has been through something similar, but unlike Reiner who has given up on that mentality and is now moving forward for the sake of his loved ones, Eren continues to advance for the sake of freedom and destroying the enemy, because that's all he has. He's no longer sure of the worth of the advance but the only way he knows how to bring about a change is through this attitude. There's even his talk with his grandfather where Dr Jäger says, "If you are of good heart and mind, return to your family, you don't want to be left full of regrets." Eren hears this and he's aware of the possible fruitlessness of his endeavour but he continues to press on. I wish the aspect of Eren finally getting beyond the hell and the mental gravity of what he had achieved was more focused upon. Did he regret it? How does he feel? Was it inevitable? I wish these questions were explored more in the finale.
Even with Gabi and Falco we see this attitude. Gabi is driven by revenge and the need to be accepted, she's going down the destructive path Eren and Reiner took before, but what saves her is Falco, who moves forward for the sake of protecting her.
As you said these opposing qualities are also embodied in Zeke vs Levi, but I'm happy Zeke was finally able to overcome it and act for the sake of Humanity in his final moments. Zeke is a lot like Reiner in this aspect, he relentlessly moves forward to achieve a goal, the goal collapses, he becomes depressed and questions the worth of striving in the first place, then he finally learns the worth and beauty to be protected in the lives other than himself.
The change in attitude is reflected in Isayama as well, in an interview when questioned about Reiner he talks about how he used to think that as long as he drew SnK he wouldn't have to worry about perfecting other aspects of his life such as social skills, but upon getting married he had to take responsibility for his family, chores and other things. He had to broaden his horizon and that change in perspective is very evident in the Marley arc, that's why it's one of my favourites.
Edit: Actually Eren was fighting for Historia's sake as well, so can we really say Eren was in the wrong? Maybe Eren is just peculiar in that manner. Even if he's fighting for someone, his super destructive side will manifest, it probably even manifests before his desire to protect. It's quite telling that he was prepared to do a full Rumbling even before Historia's life was at stake. But it's also interesting that he almost gave up that desire because it could put Historia's life in danger. He only picks it up again when it not only stops Historia from broodmaring, but is the only way he sees of protecting her at all.
I really love how the last two pages of chapter 102 of SnK really sets up how diametrically opposed Levi is to Zeke, not juts in the context of them apparently going to battle with one another, but philosophically in terms of how the characters view what’s going on around them.
We see each of them facing off, surrounded by their comrades, and while Zeke tells his fellow soldiers “Don’t let them escape. Wipe them out.”, Levi tells his fellows soldiers “Don’t you die. You have to survive.”
This really sums up in the most eloquent, economical way just what makes these two characters so vastly different from one another.
Zeke, as we know, is in truth, in this moment, betraying his own comrades, and is himself largely responsible for the current situation, having planned all of it out with Eren, and because of that, is the direct cause of so many of his fellow comrades and Eldian’s being killed. He shows no concern for Pieck’s or Porco’s well being here, only tasks them with killing the enemy, all while secretly planning on letting himself be captured. We have to remember that Zeke is seen as the leader of the Warrior Unit. He’s their captain, someone they rely on to guide them and have their backs.
On the other side, we have Levi, who’s been dragged into this fight, along with every other member of the SC, against their express will, by Eren’s independent actions, forcing them into this position of having to invade Marley in order to regain their one and only real defense against invasion. And Levi’s instructions to his squad here are the exact opposite of Zeke’s. He tells them, above all, and more important than anything, to stay alive. His number one priority is keeping his comrades safe. He orders them to survive, just like he did back in Shinganshina.
I think this is such a brilliant moment that really encapsulates why these two character’s are so powerfully opposed to one another within the narrative.
Zeke doesn’t really care about his comrades, not enough to prioritize their safety over his goals, and not enough to be honest with or trust them. He sees them as tools. They trust him implicitly, and he uses that trust to manipulate them into achieving his ultimate ends, not caring who among them pays for that with their lives.
Levi is the exact opposite, prioritizing the lives of his soldiers above wiping out the enemy, instructing them to survive, no matter what, telling them they HAVE to survive. More than anything, he wants them all to get out of there alive. His soldiers also trust him implicitly, and Levi does anything but betray that, instead going out of his way to remind them that their lives matter, and being himself with them 100% of the way, putting trust in them, and showing they can trust him in turn.
It’s a really great and important moment, I think, really defining the opposition of these two characters.