No doubt when Koro-sensei saw his students come to save him, he couldn’t be happier. Sadly, this was a trap by Yanagisawa because he knew that Koro-sensei’s biggest weakness is his students. Koro-sensei had two regrets: first, we know the feeling of being responsible for Aguri’s death.
The other is the failure of his former apprentice. As the tentacles do draw out his true feelings for Koro-sensei. It is clear he feels hatred for him because he aspired to be the assassin that he idolized. It feels poetic that Koro-sensei himself, as a teacher, experienced failure before being a part of class 3-E. As everyone in that class has made mistakes and come back from it.
Yanagisawa gets a laughable death. As an antagonist, he wasn’t good. He is the creator of Koro-sensei and simply wanted revenge because, in his eyes, Koro-sensei took everything from him. He was mostly boring; luckily, it doesn’t negatively affect the manga.
Koro-sensei has been shown to be able to do inhuman tasks. Kayano getting pierced should have marked her death. Of course, Koro-sensei didn’t use his tentacles in his fight to ensure he could save Koro-sensei. Pretty poetic because while he couldn’t save Aguri because he didn’t know how to put those skills to use. He clearly learned from his failure and saved Aguri’s little sister
Sadly a reality sets in for the students that basically life can really suck. You can think of what you should have done, but there are times you need to face the facts. Yanagisawa’s attack made Koro-sensei’s escape impossible. The students no longer have an option to save Koro-sensei. However, they have been trained to kill Koro-sensei, and with him gassed out, it is the perfect time to do it.
Of course, killing someone you deeply care about is hard. Koro-sensei has done so much for the students that it isn’t easy to just kill him. As Nagisa does struggle to do it, his teacher, Koro-sensei, gives him the push he needs. Though in a way, if anyone were to kill Koro-sensei, he’d rather it be the students that he cared so much for. At least now he can rest in peace alongside Aguri.