WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 19, "Two Brothers," now streaming on Funimation, Hulu, and Crunchyroll.
For as long as Eren Jaeger and his brother Zeke have been trying to carry out the Euthanasia Plan in Attack on Titan, Eren's friends (and the audience) have been wondering why Eren would ever go along with such a horrible plan. Toward the end of Season 4, Episode 19, "Two Brothers," this question was finally answered by the revelation that Eren had just been using Zeke to unlock the Founding Titan's power for his own purposes. While some may have seen this surprise coming, Eren was truly committed to fooling everyone, even going as far as killing innocents and betraying his friends and comrades.
It never really made sense that Eren would use the Founding Titan's powers to strip Eldians of their ability to reproduce. After all, Eren has only ever wanted freedom for himself and his friends and forcing his own people to go extinct, even peacefully, couldn't be further from freedom. However, Eren was so convincing that he had gone bad that the Survey Corps plotted to have him eaten to remove the Founding Titan from his grasp while Armin had to convince Eren's closest remaining friends that Eren wouldn't do this to them.
As it turns out, all Eren's horrible actions were done to gain Zeke's trust to the extent that Zeke would help him unlock the Founding Titan's power. Eren held the Founding Titan and Zeke had royal blood, so only together could they unleash the Founder's power. Whatever Eren's endgame is, he has let nothing stop him from accomplishing his mission, alienating his friends and allowing Paradis Island to believe he had betrayed them. Eren believed, incorrectly it turns out, that once he and Zeke unlocked the Founding Titan, he could do whatever he wanted, regardless of Zeke's participation.
Once Eren finally awoke inside the Paths after having his head blown off by Gabi, he sees Zeke shackled to the ground, apparently bound by King Fritz's will. Right when it seemed that Eren was going to get everything he wanted, Zeke revealed he never really fully trusted Eren either. Zeke pretended to be helpless just to see what Eren would do and successfully turned the tables once Eren revealed that he had been feigning cooperation with Zeke just to unlock the Founding Titan's power. Zeke's revelation that he was in full control of the Founder's power meant that all of Eren's effort had come to nothing as he was powerless to stop Zeke's Euthanasia Plan after helping things get this far.
Fortunately for Eren, Zeke refused to give up on his younger brother even after the betrayal he was already half-expecting anyway. Nobody hated Grisha Jaeger more than his firstborn son, and Zeke steadfastly believed that Eren has been brainwashed by their awful father. It's unclear what Zeke is planning to do next, but that he's going to use the Founder's power to do it does not bode well for Eren. All of Eren's scheming has come up short, but Zeke's decision to hold off on the Euthanasia Plan until he saves Eren might just be the chance that Eren needs to wrest control of the Founder away from Zeke.
The preview for the next episode indicates that Eren and Zeke will be taking a literal trip down memory lane, so a reckoning amongst the Jaeger family might just be inevitable. Eren has never given up with his back against the wall before, and as the entirety of Season 4 has shown, he will stop at nothing to gain the power of the Founding Titan. Whether Eren uses the Rumbling to crush Marley or just eliminates Titans altogether, given that this is Attack on Titan, things are probably about to get a lot worse before they get better.
