Assassination Classroom - Ending Explained

The final twist reveals that Koro-sensei was the world's greatest assassin who was experimented on by Yanagisawa after being betrayed by his student. He became a creature of antimatter and was destined to explode, regardless of his intentions. He took the job as a teacher to honor the memory of Aguri Yukimura, the scientist who cared for him. The students eventually succeed in killing him, but only because he allowed it as his final lesson. Seven years later, it is shown that Nagisa has become a teacher, using Koro-sensei's methods to help a new generation of struggling students.

Some viewers interpret the series as a political allegory for government overreach and the weaponization of children. Another perspective views the entire assassination mission as a psychotherapy session, where Koro-sensei is the therapist helping the students eliminate the toxic parts of their personalities to integrate into society.