WARNING: the following contains spoilers for Season 4 of Attack on Titan, now streaming on Crunchyroll, FUNimation, Amazon Prime and Hulu.
Season 4 of the anime is rapidly expanding the scope of the entire story of Attack on Titan, revealing the great war that the Marley Empire has been fighting against its neighbors for four years, ever since the conclusion of Season 3. Eren's three-walled city is really just a small piece of the overall map, located on Paradis Island, and this revelation is drastically changing Eren's goals and priorities.
Eren is no longer a vengeful Titan slayer, he has rescinded his "I'm going to kill them all!" oath. He now knows that all Titans are his fellow Eldians, an ethnic group that suffers second-class citizen status in the imperialistic Marley Empire. This fact has greatly warped Eren's mission, and he is hardly the same hero he was at the series' start, much to the consternation of his closest friends in the Survey Corps. This division could wind up being his undoing.
Eren's New Mission Statement in Season 4
Everyone on Paradis Island had his or her worldview changed when the reality of the Marley Empire and the larger world came upon them, and while everyone among the Scouts wants to defend Paradis Island from the conquest-minded Marley Empire, Eren Yeager is taking this to an extreme. He practically views himself as the Eldians' savior, along with his half-brother Zeke, and he is certain that with the Attack Titan and Founding Titan's powers, the oppressive Marleyans will be toppled and the Eldians will restore the old empire that is their birthright. Eren has a "we're taking back what's ours" perspective on things, and his fellow Paradis Island natives are somewhat sympathetic to this, even aiding Eren during his daring raid on Marley's capital during Willy Tybur's dramatic speech.
The Founding Titan is the key, and Eren will face anything the Marley Empire can throw at him to obtain it. He and the Marleyan leaders are two dogs fighting over one bone, and this means that the Nine Titans are split into two camps, with Eren taking the lead of his side in this struggle. If the Eldians can secure the Founding Titan's power and channel its strength with Zeke's royal blood, then surely, the Eldians will have their revenge for what the Marley Empire did to them. Right?
Eren Becomes A Loose Cannon
Eren's mission statement is clear enough, but that doesn't make that mission any easier to accomplish. It is true that Eren has many assets on his side, from the element of surprise to the Attack Titan's power and even Zeke's cooperation, but all this is straining Eren's relationship with his fellow Scouts, even his longtime friend and adopted sister Mikasa Ackerman and the no-nonsense Levi Ackerman. Eren has become a radical who cries out for Eldia's restoration, and he no longer cares how many lives are spent to achieve that dream. In earlier seasons, Eren swore that he wouldn't let any more of his friends and countrymen die at the hands of the Titans, but now, Eren sees the struggle as a game of chess. He will sacrifice any number of Scout pawns to capture the enemy king, and the Scouts soon picked up on this.
In fact, Eren more or less forced the Scout Corp's hand by launching his raid on Marley's capital, since he would surely be killed or captured if he didn't get some reinforcements. Reluctantly, the assembled Scouts equipped their ODM gear, took off in a captured zeppelin, and provided some much-needed support while Eren went on his rampage (and ate the War Hammer Titan).
Several Scouts died in this endeavor, including the beloved Sasha Blouse, and Eren hardly batted an eye. He has his mind on the mission, the dream, and who cares if a few Scouts get shot down? For now, Eren and the Scouts are on the same side, but cracks are forming in this alliance, and this may come to a head before long.
Could Eren end up taking Zeke's side against Levi and Mikasa in future developments? How will the Paradis Island's defenders ever restore Eldia if they are feuding amongst themselves? This may give the Marley Empire its chance to divide, conquer and assert its dominance once and for all, and Eren might need more than Zeke's (or even Reiner's) help to overcome those odds.