Spoilers: If you are still in the Shibuya arc, there might be spoilers for the future chapters of Jujutsu Kaisen in this article!
Kenjaku or fake Geto’s plan to seal Gojo Satoru, and his subsequent success in being able to pull it off is one of the most shocking moments of the Shibuya arc.
One can’t help but wonder how much effort was put into formulating a plan that would corner the strongest Jujutsu sorcerer alive without missing a beat. Kenjaku certainly deserves some praise for this.
However, the question still remains. Why did Kenjaku or Pseudo Geto decide to seal Gojo instead of killing him? I mean, even from the start, when Jogo was suggesting that they finish off Satoru, Kenjaku was fixated on the plan to seal Gojo using the prison realm.
Though he might have manipulated the cursed spirits in league with him, giving them false promises, Kenjaku’s motive was clear from the start, he needed Gojo Satoru out of the way, and killing him was not an option.
In fact, the entire motive of the Shibuya incident was just to seal Gojo and for Kenjaku to eventually set into motion his ultimate plans.
So what exactly was the thought process behind sealing Gojo Satoru?
Why did Kenjaku/Geto seal Gojo Satoru?
If I were to answer this in short, then Kenjaku/Geto sealed Gojo Satoru because the latter was the strongest sorcerer and would have proved a hindrance in his ultimate plans.
But this answer doesn’t even do justice to what is actually happening underneath. To understand why Gojo was sealed, we need to talk a bit about who Kenjaku is, and his goals.
Kenjaku is a sorcerer who existed over 1000 years ago, and had connections to both Sukuna and Tengen. He has been body hopping till now, possessing one sorcerer after the another, by transferring his brain to them.
This is how he came to possess Geto Suguru’s body too.
In the chapters after the Shibuya incident, we come to know that Kenjaku is aiming to merge the humans in Japan with Tengen in order to cause a forced evolution of the former. He has been trying to do this for a very long time apparently, and that is his motivation to keep on living.
You can read more about Kenjaku’s goal in the article below.
Read More: What is Kenjaku’s goal in Jujutsu Kaisen?

However, in order to achieve the forced evolution of humans he needed Tengen to not be stable.
The only way Tengen could maintain their stability and sanity was merging with the Star Plasma Vessel. If Tengen doesn’t merge with the Star Plasma Vessel, they will lose their sense of self thanks to their CT, and that’s when they will be the most useful to Kenjaku.
So, Kenjaku’s first step towards achieving his goal was to somehow stop the merger between Tengen and the Star Plasma Vessel. Naturally, the Star Plasma Vessel became his target.
Connected by fate:
Now, Kenjaku’s target may have been fixed, but, every time the vessel’s merger was about to happen, a Six-Eyes user would also appear to protect them. As Tengen mentions in chapter 145, Tengen, the Star Plasma Vessel and the Six-Eyes user were connected by fate.
So, Kenjaku had no option, but to defeat the Six Eyes User in the process of stopping the merger. This was easier said than done.

The first two times Kenjaku tried, he was defeated by the sorcerer who possessed the six eyes. This meant he had to wait for the next merger to happen before his plans could proceed.
If you are not getting the severity of the situation, then consider this; a merger only took place once in 500 years. Kenjaku’s failure to stop the assimilation meant that he had to wait 500 years before getting another chance.
So, the third time around, Kenjaku was not ready to take any risks. When the time for the merger neared, a Six-Eyes user and the Star Plasma Vessel were born, and Kenjaku killed both of them within a month of their birth.
He then decided to bide his time for Tengen to be unstable. However, when the time for the merger came, a new star plasma vessel and a Six-Eyes user appeared. The three were truly connected by fate, and Kenjaku couldn’t escape from it.
He learnt the hard way that killing the Six-Eyes user was not the right choice. And that’s how he settled on the plan to seal the next Six Eyes User and started his search for the prison realm. After all, he had another 500 years to formulate his plan for sealing.
And this time around, Gojo Satoru was the Six-Eyes user, whose job was to protect Riko Amanai, the Star Plasma Vessel.
However, something surprising would happen which would accelerate Kenjaku’s plans without even him realizing it.
A heavenly restriction and intervention:
Welcome Toji Fushiguro into the complex equation.
12 years ago, during the events of the Hidden Inventory Arc, Toji broke away from this supposed fate that Tengen mentioned, and succeeded in killing the Star Plasma Vessel.
The reason? He had no cursed energy, and that somehow help him break away from the chains that bound Tengen, Riko, and Gojo.
And to add to it, Tengen decided that they wouldn’t merge with any of the other Star Plasma Vessels who were present at that time. Instead they decided to maintain their sanity and their physical form using barrier techniques.
That, once again, was a decision which aided Kenjaku’s motives.
But then, we don’t know the exact reason why Tengen decided to not merge and stabilize themselves. After all, they knew they were a target for Kenjaku. Why take the risk of not merging, when you can just stabilize yourself and thwart your nemesis for another 500 years.
I guess it had something to do with how merging with Star Plasma Vessels seemed like a morally gray solution. Sure it helped maintain Tengen’s sanity, and also maintain the barriers that the latter had deployed helping to optimize cursed energy.
But, from the heavenly restriction arc, we know that being the Star Plasma Vessel was not the best thing to happen to someone.
Gojo and Geto had decided to let Riko go free according to her will, and that aptly describes the scenario!
In short, Toji’s intervention and Tengen’s decision had helped accelerate Kenjaku’s plan.
But it still wouldn’t move forward unless Gojo too was out of the way. So, now that Tengen had decided not to merge with any other Star Plasma Vessel, Kenjaku can just go and kill Gojo right? Right?
Well, that’s not what he decide to do!
Why not kill Gojo after Riko died?
Well, the answer rests a lot on what Kenjaku knew. He knew that Toji had interfered and stopped the merger with Riko. However, I doubt if he knew that Tengen refused to merge with any other vessel in the future.
He probably assumed that some other Star Plasma Vessel might appear in the future, and he just wanted to be ready for that situation.
If he killed Gojo, there was a chance that a new Star Plasma Vessel and a Six Eyes User appeared again.
However, by sealing Gojo, Kenjaku made sure that the Six Eyes User was completely taken off board. Tengen’s decision to not merge only made the situation easier for him.
It’s not just Tengen’s decision, even the appearance of Geto Suguru, someone who had Cursed Spirit Manipulation, only aided the Heian era schemer.

‘Why?’, you ask? Because, once Tengen evolves, he will be closer to a cursed spirit than a normal sorcerer or a human. And what better to control a cursed spirit that Cursed Spirit Manipulation.
I wonder how Kenjaku was planning on making Tengen adhere to his plan if Geto had not spiraled down a path of evil. Well, we’ll never know the answer to that. And according to me, these ‘what-if’ scenarios add to the beauty of Jujutsu Kaisen.
What if Tengen had decided to merge with another Star Plasma Vessel? What if Geto had not turned evil and died at the hands of Gojo? If any of these and other small incidents had proceeded in a different way, the outcome would have been totally different.
However, that was not to be. The situation favored Kenjaku, and he was in no mood to take any chances despite fate smiling on him brightly.
So, as he had decided earlier, he set out to find the Prison Realm in order to seal Gojo. And once he found it, he recruited cursed spirits as his allies to help him execute and complete the first step towards his goal, i.e. sealing the Six Eyes User Gojo Satoru, which is what took place at Shibuya on October 31, 2018.
TL;DR
So, to answer in short, Kenjaku decided to seal Gojo so that he won’t interfere in his plan to use Tengen to merge them with humans. And to do so, he came up with the elaborately planned Shibuya Incident after getting his hands on the Prison Realm.
What do you think of Kenjaku’s plan? If you still have any doubts regarding Gojo’s sealing, hit me up in the comments section or Tumblr.