We finally get to learn about Koro-sensei’s past. It is quite interesting that skill-wise he feels almost as unstoppable as he is in the present, except for his enhanced abilities. It is quite ironic that what resulted in Koro-sensei, the original Grim Reaper, being captured was because his disciple betrayed him. As we have seen, Koro-sensei is a phenomenal teacher in the present; he didn’t always have those traits.
The two key figures in his flashback were Yanagisawa and Aguri. One thing is for certain, and that is the positive influence that Aguri had on Koro-sensei. She was a light that he never had before. Meanwhile, Yanagisawa was engaged to Aguri, but it is clear they had no loving relationship. Kayano even states how awful he is. It’s clear he is out for Koro-sensei, as he is the monster he created. It is clear that unlike Kayano he is not doing it for revenge.
We know how the tentacles prey on your desires, but in a way they were good for Koro-sensei. As it put out his desires that he always held within. The Koro-sensei we see in the present is what he would have been if he lived a normal life outside of being ridiculously strong.
It’s interesting to learn that Koro-sensei was never responsible for what happened to the moon, but it was the result of the experiments from Yanagisawa. Of course eventually Koro-sensei would destroy the planet, so it was easy to point the blame on him for assassins to go and kill him. Sadly, the world can just be cruel at times.
It’s the ironic thing that between the monster that was Koro-sensei and Yanagisawa, Aguri deeply cared for Koro-sensei. There was good within Koro-sensei, and perhaps the tentacles brought some of that out. Which is no wonder she wanted to save him even when it seemed silly. Ironically, her death is what caused Koro-sensei to get under control. Who knows what the Grim Reaper possessing all of this power would have done.
In that moment, Koro-sensei realized he cared for Aguri through all the time he spent with her. What is the meaning of power if you can’t make use of it when you need it? Koro-sensei wanted to save Aguri but couldn’t. Though as Aguri saved Koro-sensei, her one wish was to help those students that she was teaching. We have seen Koro-sensei fulfilling that wish with what he has done with class 3-E. He used his powers to help rather than hurt. Everything Koro-sensei is as a teacher comes with the passion that Aguri had. In many ways, Aguri lives within Class 3-E. Koro-sensei has saved them and guided them just as Aguri did for him.
Now it is curious what the students do because Koro-sensei will die and either the earth with him or not. Nagisa wanting to save him sounds ideal, but is it realistic? Also, Nakamura mentions killing Koro-sensei would be the highest honor they have.