While Attack on Titan has always had a large, layered plot filled with secrets and hidden agendas, none have ever been as shocking as former hero Eren's sudden shift into full-on antagonist mode. Despite the surprise, this change has actually been a long time coming, with Eren's machinations having been in motion since the end of Season 3. However, given that we are rarely in Eren's head, his actions can seem to come out of nowhere. And with all the lore that has been added in the final season, it can be hard to keep up with how all the revelations add up to the current state of affairs. Now that his final goal has been revealed - and his ultimate move set in motion with seemingly nothing left that can stop it - here's a breakdown of Eren's plan from his perspective, giving context to all of his seemingly out of the blue actions that should show that it was actually a cohesive, coherent scheme.
Eren's plan started before the final season even began, after the Survey Corps and their allies successfully instated Historia as the queen of Paradis. When giving the corps their awards for their bravery in fighting against the Beast Titan and other Marleyans, Eren touched and kissed Historia's hand. As the Founding Titan inside Eren can only use its power when connected to someone of royal blood, which Eren is not, this kiss turned the Founding Titan on, giving him access to "Paths", the world in which we see Eren, Zeke, and Ymir in recent episodes that connects all living Eldians. With a connection to paths, the Attack Titan's ability to see into the past and future of its owners is greatly enhanced, and Eren saw visions of his father Grisha's life more clearly than ever, as well as the future he would eventually create. We didn't know what he saw at the time, but knowing now the kind of visions Eren saw then, his shift in demeanor from that point forward makes perfect sense.
With the realization that he could use a royal descendant to gain access to Paths and communicate with Ymir, Eren's goal became clear: to set up a situation where he could release all of the Titans inside the Walls, colloquially known as "the Rumbling," unimpeded. To do this, he needed access to a descendant of King Fritz and for everyone who might be able to stop him to be incapacitated, which is what he has mostly achieved now.
The first requirement, a person of royal blood, has two known living options. One of Historia, current ruler of Paradis. She is mainly ruled out because Eren has no access to her. As Paradis sets up its 50 year plan to establish itself as a world power, Historia has been hidden away for safety and so that she can have kids who can inherit Titan powers in the future to maintain Paradis' military might. Without Historia, the other choice is Eren's half-brother Zeke. While initially a problem, as Zeke has been Eren's enemy for the whole story so far, Eren eventually learns of Zeke's mass euthenasia plan through his co-conspirator Yelena, and realizes he can pretend to be aligned with Zeke to ultimately use Zeke's royal lineage for his own goals. But couldn't Eren have simply started the Rumbling earlier, when Zeke and Eren met in Marley and planned Zeke's euthanasia plot together?
That leads to the second - and more complicated - part of Eren's plan, the circumstances under which he wanted it to begin. Eren's goal is to eliminate the entire world except for Paradis, but keeping his fellow Paradisans alive is also extremely important to him, especially his close friends. After all, the elimination of his 'enemies' means nothing if the people he wants to protect aren't safe when it's all over. To complicate this, it becomes clear early on that Paradis is heading toward a goal of globalization, and those efforts are being spearheaded by people Eren particularly cares for like Historia, Armin, and Hange. He knows they wouldn't want him to go through with his plan, but he still believes it is the correct decision, so his actions for the next few years are largely with the goal of keeping them out of the picture, but still safe. He also needed to ensure that the outside world wouldn't be around to stop him at the last moment, either.
To that end, Eren's actions have all been to keep everyone away from him when he finally touches Zeke. Paradis executed an attack on Marley, but it ends up going very differently than their original plan, with important changes made by Eren to help his ultimate goal. He had Armin destroy Marley's main port and directly assaulted the audience of Willy Tybur's play, mainly military and political leaders from the outside nations, in order to cripple their ability to retaliate against Paradis in a timely manner. He consumed the War Hammer Titan to gain its versatile hardening ability, which would allow him to escape any restraint he might be put in - something he ends up putting to use when he is arrested upon arriving back at Paradis after the attack on Marley.
At home, he used his status as a symbol of power and freedom to sow the seeds of nationalism among Paradisans like Floch to create what would become the Jaegarists, putting Paradis itself in a state of internal turmoil, weakening their power structure and giving Eren a personal defense force. The Jaegarists also easily sympathized with Zeke, which allowed Zeke and Eren to meet even after being separated, which they assumed would happen as neither could be trusted after the attack on Marley went so wrong. Most distressingly, he taunted his closest friends Armin and Mikasa in order to ensure they would not follow him, then had them locked up. While Armin holds the Colossal Titan, Eren knew he would never use it to escape imprisonment with his friends around him, as they would be casualties Armin could not tolerate. With all of these goals accomplished, Eren's plan was set to go off flawlessly.
Or so it would have, if not for one main problem: Reiner. Much of Eren's plan hinged on his attack on Marley preventing the outside forces of the world from being capable or willing to fight back against Paradis right away; by both striking at Marley's forces and making a frightening show of power, he and Zeke had thought they would have taken much longer to respond. And they were almost right - Marley wasn't originally going to attack again for six whole months, which would have been much too long when Eren and Zeke's plan was already set to be carried out almost immediately. Reiner, though, saw through this, knowing that Zeke was never the kind of person to rest on his laurels, and spearheaded an immediate invasion of Paradis. This inadvertently set off a domino effect of sorts, leading to Armin and friends being released from their prison and Eren needing to fight through multiple other Titan wielders to reach Zeke.
While they ultimately were able to connect, and Eren able to initiate the Rumbling, it is hardly the ideal situation Eren had hoped for. Regardless, Eren's plan was detailed and thorough; almost every moment in the final season was in one way or another influenced by and intended by Eren to set up the moment where Eren and Zeke meet, though this can only really be seen with the benefit of hindsight. Now that we can look back at the season knowing Eren's intentions and goals, hopefully his actions make more sense than they once did. This kind of attention to detail is part of Attack on Titan's enduring appeal. After all, and it's exciting to see that the series hasn't lost its touch as it heads toward its ultimate conclusion.