WARNING: The following contains spoilers for Attack on Titan Season 4, Episode 15, "Sole Salvation," now streaming on Crunchyroll, Funimation, Hulu and Amazon Prime.
Season 4, Episode 15. "Sole Salvation," of Attack on Titan finally reveals the motivation behind Zeke's villainous plan. For weeks now we've known about Zeke's tentative plot in conjunction with Eren, and, from the end of Season 3, that Zeke betrayed his parents and the Eldian Restorationists. But we didn't know why he came up with this plan and what led him to get in touch with his half-brother or betray his parents. Now, all of these questions are finally answered in "Sole Salvation."
Zeke was born to Dina Fritz and Grisha Jeager in Liberio -- the internment camp for Eldian citizens in Marley. Zeke grew up learning that his fellow Eldians and even his own family were regarded as "devils" who committed atrocious sins against humanity over history. But his parents took the time to educate him on what they believed to be the true story of their people's origins and brought every real-world lesson back to his roots.
Over time, Zeke enrolled in Warrior training. Physically, Zeke was no match against the other Warrior candidates and fell behind quickly. His superiors and even his own parents questioned his ability to become a Warrior, which took a toll on Zeke. He soon started spending time with Tom Ksaver, the holder of the Beast Titan at the time. The more time he spent with Tom, the more the two bonded and started to agree on a lot of things, especially regarding the Warriors and Titans.
When Zeke started to fall even further behind as in Warrior training, he overheard his parents talking about how he was instrumental to their plans as an inheritor of a Titan. He quickly realized his parents were a part of the Eldian Restorationists that everyone was talking about and later learned that they were about to be captured. When he confessed this to Tom, the Warrior urged Zeke to turn his parents in. The Marleyans would look on Zeke with favor, Tom argued, as the act would show his utmost loyalty to the motherland, securing him a position as a Warrior. More importantly, doing so would save his life, as well as his grandparents'.
Zeke took Tom's advice and betrayed his parents and was indeed spared, as we already knew. But now we finally know why he did it. But how did that result in Zeke inheriting the Beast Titan when he was the runt of the pack? It turns out that Tom had some unresolved issues himself. He pretended to be Marleyan to have his wife and child but when she found out he was Eldian, his wife killed their child and then herself. All the while, Tom had been looking at Zeke as if he was his own son, confiding in him about his research and his opinions on Titans.
Tom divulged to Zeke that the Founding Titan has the ability to change the very physiology of the Subjects of Ymir, even making them immune to certain viruses. The Founding Titan could therefore make it so Subjects of Ymir cannot bear children, effectively wiping out the Eldian race. Due to Zeke's own suffering, and the knowledge that most Eldians he knows only endure suffering, Zeke agreed with Tom's desire and decided to make it happen -- to "euthanize" all Eldians so eventually Titans would be extinct, as there would be no one to carry the gene.
When Tom's time came to pass on the Beast Titan, Zeke was chosen because he pledged to carry out Tom's wish. It didn't help, either, that he's got a pitcher's arm. Tom and Zeke spent most of their time playing catch, and Tom would know better than anyone that the Beast Titan could use that skill well. In addition, Zeke's betrayal of his parents proved his loyalty to Marley, as intended, further securing his position as a Warrior.
Even though we now have Zeke's full origin story and the why behind his villainous plan for Eldia, it still doesn't rationalize the brutality of his plan. Nor does it excuse Eren being on board with it. The stakes are the highest they've ever been, and there's only one more episode left to conclude Attack on Titan. Will Zeke's plan be fully realized? Or will another path be forged that saves both the Eldians and the world?