WARNING: The following contains spoilers for Season 4, Episode 15 of Attack on Titan, "Sole Salvation," now streaming on Crunchyroll, Funimation, Hulu and Amazon Prime.
Over the course of its four-season run, Attack on Titan's villains have gone from being mindless, easy-to-kill monsters to the same flesh and blood as those that do the killing. But even with these commonalities, few figures have remained as clear-cut in their villainy as Zeke Jeager. That's not to say that Zeke isn't entirely without sympathy, and Season 4's Episode 15 not only shows the Beast Titan in his most sympathetic light yet -- it simultaneously compounds precisely how, and why, he absolutely must be stopped.
Now that we know wine infused with Zeke's spinal fluid has been distributed among Paradis' military, it's clear that Zeke, Yelena and the Jeagerists are planning something nasty. But that's not even the worst of what the Beast Titan has in-store. As Zeke stares death directly in the face -- in the form of a Thunder Spear set to blow his body apart by Levi Ackerman -- "Sole Salvation" takes some time to flashback to Zeke's troubled childhood in Marley. There, we're given much greater insight into why he sold his parents, Grisha and Dina Jeager out as Eldian Restorationists to the Marleyan military police (a death sentence Grisha would escape). Trapped between his Marleyan loyalist grandparents and his revolutionary parents, the conflicting doctrines, and pressure to become a Warrior candidate, led Zeke to find guidance outside of his own blood in Tom Ksaver, the then-holder of the Beast Titan.
Tom was the one who instructed Zeke to turn his parents in to save himself, after which he became a surrogate father to the young man. But a Titan Shifter power -- and a love of baseball -- weren't the only things Tom passed down to Zeke. As a Titan scientist, he revealed top-level secrets about the creatures to his one-day inheritor, most crucially, how the Founding Titan's hindrance can be overcome, and how someone who controls the Founder can use its unique power to alter the genetics of every Subject of Ymir via the Path. To back up his claim, he cited a disease that ravaged the Eldian community hundreds of years ago, only to be suddenly eradicated by their Fritz monarch (in possession of the Founding Titan).
From this key revelation, and with resentment towards Grisha and his own Eldian blood clear, Zeke suggested that this ability might be used to prevent Eldians from reproducing -- something he referred to (erroneously, we might add) as "euthanization." Tom was shocked, but unfortunately, not for the reasons viewers would hope. Having lost his wife to suicide, and his child to her own hands beforehand, Tom became so disillusioned with Eldian existence that he agreed with Zeke's idea. And this goal is what Zeke has been pursuing this entire time -- even before inheriting the Beast Titan. It's likely that he has no intention of going ahead with the Rumbling test. Instead, he merely wants contact with Eren so that the two of them can bypass the restrictions on the Founding Titan and enact the euthanization plot.
Worse still, another flashback to when Eren and Zeke secretly met in Liberio, prior to the raid, uncovers the extent of Eren's own collusion. Zeke's younger brother seemingly concurs that the only way to truly "free" their race... is by ending it. These are incredibly dark themes, even for Attack on Titan, and fans may have a hard time reconciling with Eren and Zeke's motivations. Eren's point of view comes from a life largely spent hating and killing his own, unknowingly, and even after discovering that there is a wider world beyond the walls, realizing that much of it despises his people just as much as he once did. Zeke, meanwhile, was raised in a state of systematic Stockholm Syndrome, in a manner of speaking. Marley teaches its Eldian citizens that they are subhuman thanks to the crimes of their ancestors -- with their only value lying in their Titan-ready genes, turning them into either zombie-like Pure Titans or live out a 13-year existence as a Shifter. Either way, they'll be forced to kill other Eldians, and be grateful for it.
Understanding this doesn't make Zeke at all justified in the mass sterilization of his own kind. In fact, it's exactly in-line with what Marley's real-life inspiration aimed to do for Eldia's real-life inspiration. Zeke may think it's a more humane option, that he's "saving" future Eldian children from a lifetime of suffering. All he's really doing is giving his oppressors exactly what they want: a world without people like him in it. Little does Levi know that Zeke choosing to blow himself up at the penultimate episode's end may have actually saved countless others from never even existing.