Attack on Titan was a classic shonen tale during its early days, featuring protagonist Eren Yeager as a fierce underdog ready to take on all odds to protect his friends and countrymen (complete with a shonen rival to push him harder). That's a prototypical start to the "hero's journey," but as the series progressed, it's proven to have had a few tricks up its sleeve. By now, in the show's fourth and final season, Eren is something else entirely.
Eren's original intent to destroy all Titans has given way to his radically nationalist feelings about the fallen Eldian Empire, and he will partner with anyone -- including his traitorous half-brother Zeke Yeager -- to bring back Eldia and crush the Marley Empire once and for all. Altogether, he's becoming harder and harder for fans to root for, or justify being a fan of. Eren is a spectacle to behold, but is he totally sympathetic? Maybe not. But perhaps that's not a bad thing.
Eren Yeager's Idealistic Beginnings
It could be said that Eren Yeager's character arc can be split into two major phases, with the first phase being the longer of the two, and it follows traditional shonen patterns. Eren grew up as a tough but reckless kid needing his "big sister" friend Mikasa Ackerman to look after him when he got in over his head with bullies. Mikasa continued to look after Eren as they got older, using her astonishing combat skills to keep Eren safe against myriad Titan threats. Eren also has the stock "weak but smart" friend Armin Arlert, who deeply valued Eren's friendship. Eren was burning for revenge for his mother's death, and he trained hard with the other cadets, from Mikasa to Sasha and Connie, to become a proper Scout and fight the Titans properly. So far, so shonen.
For a time, Eren's experience in Attack on Titan resembled a hybrid of mecha action and zombie survival horror, and he unlocked another shonen trait partway through: the "hidden monster inside." The Attack Titan. Eren trained and practiced with that Titan to better protect his friends, and he learned much in mortal combat against Annie's Female Titan and Reiner's Armored Titan.
By the time Season 3 ended, the Titan threat was all but vanquished, including Rod Reiss's wild ambitions with the Founding Titan, and the Scouts learned the truth. The Marley Empire was out there, and it was the true enemy of Eren and his friends. By now we know that the inhabitants of the walled city live on Paradis Island, the home of the exiled Eldians, and an international conflict has been brewing. This has come to bring out the worst in Eren, transforming him into a character fans are recognizing less and less.
Eren Yeager, The Colossus Of Vengeance
Attack on Titan's story is intricate and unpredictable, often shocking viewers with the deaths of major characters or shadowy conspiracies. The transition to Season 4 kicked all this into overdrive, with the narrative shifting from "desperate humans vs Titans" to international war and a new perspective on what the Titans actually are. The Titans are not just flesh-eating giants; they are all Eldians; ultimately, the victims of Marley's determination to use Titans against their enemies via the Titan serum. The people of Paradis Island are gearing up for a new war against a new enemy, and the radical Eldians, including Eren, are feeling violently patriotic about it. There is some sympathy in this.
Eldians live both on Paradis Island and in the Marley Empire, with the latter being second-class citizens in internment zones. Eren is justifiably furious about this, and he is determined to free all Eldian people from Marley's tyranny and usher in a new Eldian empire, just as Grisha Yeager and Dina Fritz intended. But to do this, Eren is rapidly becoming a loose cannon, seizing any and all power that will aid him on his quest and using it to explosive ends. He launched a daring raid on Marley, forcing Mikasa and the other Scouts to come along and back him up, despite the risks (and indeed, Sasha lost her life to Gabi). What makes him especially hard to root for anymore is the fact that Eren doesn't care; he wants results. He even ate the War Hammer Titan and threatened Hange Zoe and the others if they dare resist him.
As of the most recent episode, it's clear Eren isn't even loyal to the walled city's military or Queen Historia; he is pledged to the Eldian nationalist cause, and his only allies are those who feel the same way, such as Zeke and Floch. None of this makes him any less interesting, though. In fact, it might actually make him more so. Eren is no longer the sympathetic, scrappy underdog walking the noble shonen path. He is now a spectacle who shakes the world; compelling not because of who he is, but what he is doing. That's what it takes to resist Marley, in his mind, but it may cost Eren his humanity and his likeability. So far, it seems he's more than willing to pay that price, even if his longtime fans are horrified by the sight.