WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 21, "From You, 2,000 Years Ago," now streaming on Funimation, Hulu and Crunchyroll.
There are no two ways about it now: Eren Jaeger is the villain of Attack on Titan. Season 4, Episode 21 finally revealed why Eren wants the power of the Founding Titan, and Grisha Jaeger was right that it's worse than anyone could have imagined. After Eren activated the Rumbling, Armin Arlert was the first to notice that all the island's walls had come down. At the same moment he put it together, Eren used the Founding Titan to speak to the Subjects of Ymir and announced his intention to wipe out all life outside Paradis Island.
At the end of Season 3, Eren pointed across the ocean and wondered aloud if he killed his enemies over there, would he finally be free? As it turns out, he plans to do just that. The Eldians are despised around the world for their ability to become Titans, and for the countless atrocities committed by Titans over the past 2,000 years. In fact, hatred for Eldians is the one thing the entire rest of the world in Attack on Titan actually agrees on. Even as Eren and Zeke struggled for control of the Founding Titan, a Global Alliance was forming in Marley to strike Paradis Island in retaliation for Eren's attack on the Liberio Internment Zone.
Earlier in Attack on Titan Season 4, the military on Paradis Island agreed that their best option was to allow Eren to activate a sample of the Rumbling with a small section of Wall Titans to defeat the Global Alliance. This plan would have allowed Paradis Island to maintain its security as it caught up to the modern world in terms of technology and warfare, while the other nations recovered from losing their military to the proposed mini-Rumbling. While this still would have resulted in an astronomical number of casualties, it's nothing compared to what Eren plans to do now.
Eren saw a great deal of the future when he kissed Historia's hand in Season 3, but as Zeke pointed out in "From You, 2,000 Years Ago," he clearly didn't see everything. Still, it's unclear what Eren saw that has pushed him to this point. After all, his plan is a genocide, unjustifiable even considering the world's hatred for Eldians. The innocent lives taken would vastly outnumber the lives of those who would actually take action against Paradis Island in the first place. Armin's brief joy that Eren was still truly on their side quickly turned to ash when he realized Eren is officially on no one's side but his own.
Mikasa was horrified by Eren's actions when he only destroyed a small section of the Liberio Internment Zone, so it's difficult to imagine her supporting Eren now that she knows what he's going to do. As it turns out, the surviving members of the Survey Corps will have to decide once again if they're going to back Eren, and this time it feels unlikely he will keep their support. While even pacifists like Armin will accept bloodying their hands in the heat of battle, Eren's plan to wipe out life around the globe is beyond unnecessary to ensure Paradis Island's safety.
Instead of proving to the world that Eldians don't deserve to be hated, Eren has ensured the hatred will never end -- even if only a single non-Eldian manages to survive the Rumbling. As the millions of Colossus Titans begin their march alongside Eren's gargantuan new Titan form, the world may be powerless to stop him just as Willy Tybur predicted. Zeke Jaeger is the only one who knows just how far off the reservation Eren has gone or has any hope of stopping him, and Eren's first act upon receiving the Founding Titan's power was to capture his brother. Mikasa, Armin and the rest of the Survey Corps may not be able to do anything but watch as Eren wipes out all life beyond Paradis Island.
