WARNING: The following contains spoilers for Chapter #139 of Attack on Titan, "Toward the Tree on That Hill," by Hajime Isayama, Dezy Sienty and Alex Ko Ransom, available in English now from Kodansha.
Even before Attack on Titan concluded, Eren Jeager was already one of manga and anime's most complicated protagonists. From humble origins inside the walls, Eren steadily went from an idealistic boy emboldened by the death of his mother and the destruction of his home district, to the figurehead of an extremist, nationalistic movement, to the greatest villain his world had ever known. For most of the manga's latter arcs he was a closed book to both his closest friends and fans of the series -- his true motives held tightly to his chest.
The last chapter, "Toward the Tree on That Hill," finally draws the curtain back on this subject, and the confessions that pour out of the wayward lead contextualize his actions in ways that are both sensical for his character but no less disturbing to readers.
These revelations come out during a flashback sequence at Chapter #139's start. The series returns to the previously unseen conversation Eren and Armin had via the Path when the latter was en route to stop the Rumbling on the Azumabito vessel. When Eren discloses the future he saw at the medal ceremony, Armin realizes everything his childhood friend has done has been in service of making that future happen. Moreover, by initiating the Rumbling, Eren willingly turned himself into everything Eldia's enemies accused him of being: a dangerous rebel intent on crushing the Island's enemies.
By comparison, those who went after him -- the ragtag alliance formed between Eldian and Marleyan soldiers, most of whom were Eren's dearest confidantes -- would be made to look like the very definition of heroism, putting personal loyalties aside to save a world that had branded them as "devils," and spent decades attacking them with their own kind.
Eren also knew that these actions could rid the world of the Titan curse, the cause of so much bloodshed and persecution for two millennia. While he didn't know exactly why, he learned that the outcome of a "choice" Mikasa had to make had the potential to free the Founder, Ymir, the 'mother' of all Titans, from the slavish devotion to her husband, King Karl Fritz, that kept her and her monstrous progeny bound to a tormented existence. In the end, that choice was to kill Eren.
Overall, Attack on Titan's story, and Eren's legacy, leave audiences with a classic 'ends justifying the means' moral dilemma. Knowing that a largely positive outcome for the Subjects of Ymir -- and those he loved -- was achieved doesn't erase the fact the Jeagerist martyr committed genocide to get there. In fact, had Eren been framed a little differently, his arc would be that of a typical tragic villain's -- Anakin Skywalker's transition to Darth Vader comes to mind. What anchors Eren to a more antiheroic role, however (though it's a pretty weak anchoring), is his self-awareness. Eren's commitment to villainy was never genuine, even if he sold it as such painfully well to those around him.
While talking to Armin in Chapter #139, Eren more or less has a complete breakdown over the crushing weight of what he'd felt forced to do. The combination of both the Attack Titan's past and future sight and, more recently, living in the timeless void within the Founding Titan's subconsciousness collapsed his sense of time and space. It's not hard to understand how having access to multiple timeline points at once could loosen one's grip on reality. And Eren was hardly a passive observer, either. In past chapters, he demonstrated to his brother Zeke that he could actively interfere with certain key events in the past, including -- and most shockingly -- talking their father, Grisha, into stealing the Founding Titan from the royal family to ensure he would one day entrust it to his then-unborn second son.
The final chapter adds yet another startling rewriting of history to Eren's list of crimes: preventing Dina Fritz's 'Smiling' Titan from eating Bertolt during the Warriors' initial attack on Shiganshina, sending her, instead, to consume Eren's mother.
While revealing this detail to Armin, Eren's excuse is that Bertolt wasn't "supposed" to die that day, implying multiple timelines may theoretically exist in Attack on Titan. We could also infer that Eren knew, from his present-day position, that his mother needed to die in order to give his past self that all-important, Titan-hating motive. In this regard, his arc is something of a self-fulfilling prophecy. Similar to Doctor Strange scouring 14 million timelines to find the best win scenario for the Avengers in Avengers: Infinity War, Eren, with the Attack Titan's ability and contact with Ymir's bloodline, found his win scenario, and remained as tight-lipped about it as the Doctor did upon his resurrection in Avengers: Endgame in order for those around him to make the decisions they did.
It should be noted, when considering the morality behind Eren's decision-making, that this doesn't mean his win scenario was the only available one, or the best. Eren's personal quest for freedom, rather than what might be best for the wider world, guided his every move, no matter the sacrifice to achieve it. Though, as a byproduct, the world may well be arguably better off in the long run, the fact remains that his mission was carried out alone, and by systematically removing freedom of choice (and sometimes life) from others. He dies as a villain to most and a hero to few but more accurately, perhaps, as the closest thing Attack on Titan had to an amoral god.
The penance for this, in the end, is not being allowed to reap the benefits of the Titan-less world he creates, which is what Eren is the most remorseful about in the final chapter. Many readers have expressed disappointment over his death but the writing was on the wall for a long time. Much like Death Note's Light Yagami, redemption for antiheroes who commit atrocities this severe would be a very contentious turn to pull off, and a peaceful retirement is neither realistic nor narratively sound. Though expressing guilt is not atonement enough, at the very least, Eren fans may find a modicum of solace in his aforementioned self-awareness, and in a love for his friends that was so fierce, the loss of 80 percent of humanity was a price he was willing to pay for their survival.