An Attack on Titan season 4 Easter egg may confirm the real reason Eren became a villain, and how he intends to use the Founding Titan's power. Eren of Attack on Titan season 4 is a far cry from the bright-eyed protagonist who joined the Survey Corps as a young boy. Unwilling to accept more generations of oppression in which Eldians continue shortening their lives in the name of becoming Titan-shifters for their country, Eren has taken direct action by partnering with half-brother, Zeke. Eren possesses the Founding Titan; Zeke carries royal blood, and together they can harness the full, godlike power of Ymir.
Zeke Jaeger has made his intentions perfectly clear - activate the Founding Titan and remove Eldians' capacity to reproduce, essentially euthanizing the entire race. Eren claims he's working toward the same goal - or that's what he's telling the Zekealots, at least. Whether Eren really shares his brother's vision remains ambiguous, and in Attack on Titan season 4's "Sneak Attack," Armin openly questions whether his oldest friend (even his oldest friend turned rotten) would forcibly sterilize a whole population. A subtle background detail in the same Attack on Titan season 4 episode proves Armin is right to be skeptical... but hints at an even worse outcome.
Changed by their recent adventures in Eldia, Gabi and Falco reunite with the Marleyan army. Together with Falco's older brother, the kids take refuge in some kind of pet store, where Gabi overhears Sasha's family walking past outside, worrying about her despite knowing she fired the bullet that killed Attack on Titan's resident potato-muncher. Gabi experiences something of an epiphany upon hearing their kindness, as everything she's witnessed since arriving in Eldia hits at once. "There weren't any devils," Gabi acknowledges. She and the other Marleyan warrior candidates were lied to, conditioned into believing the folks of Paradis Island were pure, unadulterated evil, just as the Eldians assumed the Titan-shifters who attacked them were pure, unadulterated evil. As Gabi utters these words, Attack on Titan's roving camera focuses on the shop's wide selection of cages surrounding the trio.
It's no coincidence that Gabi delivers her "There weren't any devils" speech in a shop filled with different kinds of cages - it's a symbolism that illustrates her point. Gabi realizes Eldia and Marley have become trapped within the same cycle of hating each other. Both sides are oppressed, whether its the three walls of Paradis Island, or the grim Nazi Germany-inspired surrounds of Marley's internment zone, but though those cages don't look the same, they're still cages. Gabi's revelation - combined with the symbolic pet cages of all shapes and sizes - points Attack on Titan viewers toward the true intentions of Eren Jaeger.
Ever since Attack on Titan began, Eren's overarching aim has been to find freedom, both for himself and his closest friends. Zeke's euthanization plan would simply be swapping one cage for another. A cage where Eldians aren't free to choose their futures, or start families, or pass on their legacies. Sure, Zeke's idea liberates Eldia from the cycle of war, but his alternative is like picking out another variety of cage to purchase from the selection in Gabi's pet store.
Eren's secret plan likely involves smashing the shop completely and setting fire to the ruins (figuratively speaking); removing each and every cage that could possibly trap Eldia for the foreseeable future. Armin believes Eren will achieve this by activating the "Rumbling" - unleashing the Colossal Titans housed within Paradis Island's three walls. Through this show of force, Marley may be too scared to retaliate... but that's what everyone thought would happen after Eren's attack on Liberio, and now Attack on Titan season 4 is in the midst of a full-scale invasion. How far is Eren willing to go in ensuring his friends become free as birds...?
Attack on Titan season 4 continues every Sunday on Funimation, Crunchyroll, and Hulu.