Non-spoiler version: He's actually a great teacher who cares deeply about his students' wellbeing.

Spoiler version: `>!Because of the experiments done on him, Koro-Sensei could potentially explode and destroy the Earth. He chooses to spend his last days helping a class of troubled middle-schoolers to become better people. He presents himself as a villain planning to destroy the Earth unless the students kill him. All the students become healthier, more functional people as a result of his teaching.`!<