Zeke’s proposed plan of euthanizing the Eldian population to end their troubles seems extreme, but Eren shockingly went along with it. Why did Eren agree to Zeke’s plan, and does he have his own agenda?
Eren chose to agree with Zeke’s plan because he wanted the Founder’s power. He played along until the two made physical contact and were transported to the “coordinate” location. There, Eren admits that he’d never consider euthanizing Eldians and only played along to gain the power for himself.
Although Eren’s own plans doesn’t quite work out how he intended, he was able to convince Ymir to join him. She gave access to her full array of abilities, including The Rumbling.
Chapter Summary Behind Why Eren Chose to Agree With Zeke’s Plan
After royal-blood-haver Zeke made physical contact with Eren, the two were able to access “the coordinate where all paths converge.” The other-wordly location was where members of the royal family who inherited the Founder’s powers came when they made use of it.
Zeke explains that time within the coordinate realm moves far slower than the real world. He remarks that it feels like years have passed alone in the seemingly short time it took for Eren to recover and “wake up” within the space.
Despite Gabi seemingly ending Eren with a sniper shot, Zeke was able to catch his brother’s head with just enough time to activate the transportation to the coordinate area before Eren’s life fully faded.
At this time, Ymir reveals herself. Zeke explains that her presence alone is enough evidence for her identity. Now is Eren’s opportunity to command Ymir to fulfill their euthanization plan.
You want me to euthanize every Eldian? There is no way I could ever go along with a plan that messed up. Sorry, brother. I was just playing along so I could get here.
Eren to Zeke | Chapter #120
Eren reveals the true reason behind agreeing to Zeke’s plan: access to Ymir. He wanted to work with his brother to access the coordinate realm which lets the Founder’s powers flourish. Eren purposely lied to his brother so that he could have the opportunity to use the power as he saw fit.
Zeke pleads with Eren that so long as Eldians exist, the “hell” of being burdened with the stigma of transforming into titans will never end.
Zeke Reveals He Assumed That Eren Could Lie
If you don’t do this now, the bloodshed will never end. What we’ve repeated so many times will continue for eternity.
Zeke to Eren | Chapter #120
Eren ignores his brother’s plea. He says that because he “was born into this world” he reserves the right to protect his people and follow his own will to use the Founder. Instead, Ymir ignores Eren’s request for her power and Zeke reveals that the interaction was a test.
In the “years” that it took for Eren to wake up within the coordinate, Zeke learned that his royal blood allows him to take advantage of Ymir’s ability to create anything she wants. He breaks the seemingly impenetrable chains that held him to the ground, standing up in front of Eren.
Zeke explains that he learned how to nullify the “vow to renounce war” that prevented previous royal-blood-wielders from fully tapping into the Founder’s powers. He was able to do this because unlike the kings of the walls throughout history, Zeke arrived at Ymir without being tainted by the first king’s ideology.
Ymir completely submits to anyone with royal blood, believing them to be her master. Zeke took this power and admitted to Eren that he was nothing but the key to her. It was not the other way around that Eren was led to believe. Zeke explains that he waited to tell Eren because he wanted to test if he was still “brainwashed” by their father. He does not want to use the Founder’s power until he can convince Eren to join him. He then proceeds to use the power of Paths to walk with Eren through their father’s history, hoping to convince Eren along the way to join his side.
So that’s the immediate answer. Eren chose to agree to Zeke’s plan because he wanted access to the Founder. There, he planned to take the power for himself.