The following events contain heavy spoilers from AoT manga. Please proceed with caution.
From the Marley Arc, we get to know that Zeke has actually betrayed Marley with the help of Eren and the Survey corps in order to take the first steps in his “secret plan” to take motion.
During the meeting with Darius Zackary and the others, discussing the conditions presented by Zeke, Eren misunderstands the intentions of his half brother as “the Rumbling” which is meant to free the island dwellers from their suffering.
But after Zeke revealed his plan, Eren was quite surprised by his brother’s ideology. However, he knew that possessing the Founding titan, the control is actually in his own hands, while Zeke will just be a necessary catalyst. That is why he kept agreeing to Zeke’s plan until the very end like an obedient brother.
As soon as Gabi sent flying Eren’s head, Zeke held out his hand and caught it making the “contact” he wanted so much to happen all these years.
Finally, the brothers enter the coordinate and Zeke explains everything he knew about Ymir. But little did he knew, that Eren had also been waiting for this moment for a long time just to turn the tables on Zeke, and overwrite Ymir’s will to serve the Royal Family, setting off ” the Rumbling”.
So, why did Eren betray Zeke?
Eren betrayed Zeke because their method of saving Eldia and their ideas of freedom were poles apart.
This betrayal was not something that happened out of the blue. If we look at Eren’s and Zeke’s personalities closely the answer was evident.
Eren being a fighter, could not come to terms with the Euthanisation plan that Zeke put forward. According to Eren, he was fortunate enough to be “born into this world.” And that was a choice he did not want to take away from the people of Eldia.
Its because…I was born to this world.Eren yeager
The difference in Ideologies:
Eren was a fighter from the start. Possessing the Attack Titan, further pushed him to pursue his dream to achieve freedom at any cost.
Even though he begins as a pretty one-dimensional character- someone who was wronged by the titans but then he wanted revenge; circumstances forced him to realize that the world is never as monochromatic as we like to think. It’s never a case of one side being completely pure while the enemy being utterly evil and sinful, that is just not how life works.
When he finally found a way in which he could save Paradis, it came with the price of killing thousands of innocent people. But he thought it was the only way to restore Eldia and its glory, and he was absolutely fine to take upon the role of the devil.
On the other hand, Zeke, Eren’s older half brother also wanted to free Eldia. But he wanted to do so without a fight. He wanted the Eldians to accept their fate and be extinct. His plan was to carry out Tom Xaver’s ideology to free the Eldians from the continuous suffering by stripping off their freedom. Basically, he wanted to Euthanize the whole of Eldia.
Unlike Eren, Zeke was not a fighter. This reflected heavily in the choices he made, even betraying his parents in the process.
Instead of standing up to the people who looked down on him, Zeke was someone who wished he were never born. This is what made him lean towards the Euthanisation plan. In fact this was the very motif that sparked this plan into action.
That’s why, even though he knew that in the near future the Titan tactics will be overpowered by modern weapons and artillery, he still wanted the power of the Founding titan inherited by Eren.
Zeke believed his plan would bring peace to the world because he feels that Eldians due to their ability to turn into Titans will always be condemned to suffer in this world. They will always be mistrusted by other nations and their titan powers will always bring conflict with everyone leading to an endless cycle of death and suffering.
There’s no way a fighter like Eren would have agreed to backing down meekly in the face of disaster. There’s no way he would have turned his back on the naysayers who called him devil. Eren was fixated on his goal and there’s no way he was going to budge.
So, if he was fully aware of Zeke’s futile and unfair plan for Eldia, why did he decide to go along with it at the beginning?
Why did Eren side with Zeke?
Long answer short; Eren sided with Zeke because he wanted to make use of the Founding Titan’s powers. This was only possible if he was in contact with someone who had royal blood. Eren did not want to pull Historia into the mess involving Titans and its powers, and so Zeke seemed like a logical choice.
Who was Zeke to decide the death of an Eldian? Even though at a certain point Eren felt Zeke was right, he vehemently rejects his idea.
It is just like Grisha did with Zeke, pressurizing his own philosophy on his son and stripping off him from the freedom to choose. Zeke is also making the same mistake by taking away the freedom of the Eldians to choose if life is worth living despite the suffering.
Eren always sought freedom for himself and the people he cared for. He wanted to use the powers of the Founding Titan to win their freedom.
But he realised that the powers of the Founding Titan lacked a necessary trigger to complete his plan for “Rumbling”. It is then he decided to sarcastically play along with Zeke until he reached the coordinate and understood the power he possessed.
Let us know in the comments, which side of justice do you prefer- Eren’s or Zeke’s? Do you agree with Eren’s idea of causing rumbling to win freedom for Eldia? Let us know in the comments section!