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.
Attack on Titan's concluding chapter offers a number of revelations, both big and small: from Eren's admission of unyielding love for his friends (Mikasa, especially) to the discovery that Ymir Fritz's own love for her husband is what kept the Titan curse alive for 2,000 years -- only to be ended by Mikasa's slaying of Eren in the penultimate chapter. But those aren't the most explosive details "Toward the Tree on That Hill" uncovers.
The most shocking thing of all is laid bare on a single page, and, what with everything else going on, is easily buried. But it really shouldn't be because the implication therein is yet another example of the horrid lengths Eren Jeager has gone to in the name of freedom.
The moment in question occurs within Chapter #139's introductory flashback sequence. In the sequence, the full conversation we didn't know Eren and Armin had via the Path unfurls, which Eren erased from his friend's memory to make sure the knowledge didn't interfere with Armin's decisions in the coming battle: stopping the Rumbling, initiated by Eren. The memory, however, returns to Armin when Eren dies, and out of all the expositionary things Eren tells him -- his reasoning for deciding to wipe out 80 percent of the world and why he pushed his friends away -- the one that gets the biggest reaction from Armin is that Eren was, at least in part, responsible for his own mother's death.
As Eren discusses how the Founding Titan's full power has collapsed his perception of the past, present and future, forcing him to experience them all at once, he alludes to the inciting incident at Attack on Titan's start which set him on his Titan-killing path: the moment Carla Jeager was eaten by the Smiling Titan, aka Dina Fritz, his father's first wife from Marley. In the visuals that accompany Eren's words, Bertolt, the then-holder of the Colossal Titan, looks around in horror amid the devastation in Shiganshina caused by his and the other Warriors' invasion. We only see Dina's Titan form from behind, but Eren's narration makes the situation clear. "That day, that time... It wasn't Bertolt's time to die yet. The one who let him go and made her go that way was..." Eren trails off but Armin's wide eyes silently acknowledge he knows what Eren's admitting to. He doesn't allow him to dwell on the thought, either; a quiet mercy, perhaps.
Though we don't find out how Eren was able to help orchestrate his mother's death in the chapter, the answer most likely lies in his prior time-hopping manipulation of his father, Grisha. While revealing his true intentions to Zeke Jeager, the brothers journeyed -- like Dickensian ghosts -- into their father's past via the Path. Zeke tried to convince Eren that the Eldian radicalist was an abusive father but Eren turned the tables on him: instead, actively goading Grisha from the present-day into going through with his plan to steal the Founding Titan from the Reiss family. Using the Attack Titan's power, Eren saw his future self do all this when he kissed Historia's hand during the medal ceremony, which is the moment he knew what he had to do to ensure that the future he saw -- a world without Titans -- could come to pass. Clearly, he deemed Bertolt's survival to be a key piece of this. If Dina had eaten the Warrior that day, it might not have been passed onto Armin, who couldn't have then used it against Marley and later in the world-changing Battle for Heaven and Earth.
So there you have it: airtight proof that the Jeager family is one of the most messed-up in manga and anime history, and that Eren's commitment to killing all Titans really did know no ethical bounds.