I’ve been enjoying “Attack on Titan” The Final Season, and one of the most notable changes from Seasons 1-3 and this one is the fact that series protagonist Eren Yeager (Bryce Papenbrook) goes from being the series’ protagonist to its antagonist, but as recent episodes have revealed, not much has really changed about Eren’s character; his nature has remained constant since the day he was “born” — the day he killed in order to save Mikasa. The biggest difference between Seasons 1-3 in terms of his circumstances is that in those seasons, Eren and company are on the defensive, while Season 4 sees Eren initiate the conflict, in practice being on the offensive the entire series.
He is more radicalized in Season 4, as he has had years to ruminate on what to do to save everyone in the walls on Paradis Island. The fact that at the end of Season 3, he foiled Marley’s plan to kill everyone in the walls no doubt played a role, as well as the ability of the attack titan he possesses — to see into the past and future, which is corrupting (it’s the same power Dr. Manhattan has in “Watchmen”, and the reason why he’s an amoral character), but I don’t think had a major impact in his decision to destroy his enemies outside the walls.
I do, however, think that the attack titan’s power is the reason why he decided to go a step further in this week’s episode, in which he declares that he will use the founding titan’s power to destroy all life outside the walls, as it’s a huge overstep and makes him completely irredeemable, but his insight into the future essentially puts him onto a path of destruction he cannot escape. In fact, if you’re familiar with how the manga ends (which I will only discuss broadly here), I think it’s arguable that Eren is intentionally becoming a villain that is meant to be struck down for the greater good, much like how Zero is eventually struck down at the end of “Code Geass”. Eren has molded himself into an agent for radical change, and there is no going back after everything he’s done. Even if the Curse of Ymir was not going to eventually take his life, he’s also something that can’t exist or function in peacetime.
Looking back at Eren’s journey, even if he didn’t have the ability to see into the future, I’m not seeing any scenario where he’s not a villain, because he won’t stop until everyone inside the walls is safe, and the only way he can do that is to retaliate against Marley. In Eren’s mind, there’s no way the two can coexist — at best, a strong message can delay an invasion from the outside like King Fritz did, but the only way to stop it is to take out his enemies.
The power of the titans ultimately is a curse that’s not worth the power it grants, and the quickest, and perhaps only, option for Eldia is to join the outside world, which is impossible to do given how the world is united against them due to their past use of titans in war, though it’s worth noting that Marley has also committed atrocities using titans. But as long as Marley is strong and has it out for them, they won’t be able to form alliances with any nations that can guarantee their protection, though destroying Marley’s allied forces should be enough to shake their grip on the world.
The power of titans, nonetheless, would always pose an issue, though it’s alluded in Season 4 Part 1 that industrialization and advances in warfare are slowly making them obsolete.
Eren was always going to be a villain, especially when Marley’s true intentions on Paradis and feelings towards Eldians were revealed. They’re not exactly an enemy you can bargain with, as they are driven by bigotry they think is justified, and their devastating attack on Paradis in Seasons 1-3 and continued aggravation forced the islanders to respond. While there are more nuanced ways Eren could’ve handled the situation, his past trauma and literal ticking clock via the curse of Ymir that will cut his life short makes it so that he’s either unable to see those paths, or lacks the time to go down them.
This doesn’t justify or endorse any of his actions, but from Season 1, this is the path that he was always on.