Explaining all of it would be a long story... [Nagito hums thoughtfully as he considers a summary.] There was a teenage girl who felt nothing but despair. She wanted to spread that despair to the world. And, unfortunately for the world, she was very good at it! [He's trying to sound cheerful, but his obvious loathing for the girl in question seeps in, and he just sounds extremely passive aggressive.] She gathered others willingly or unwillingly and caused The Biggest, Most Awful, Most Tragic Event in Human History. Or just the Tragedy, for short.

Now, Naegi-sama, he was actually the girl's classmate! [And now, as soon as Makoto enters the picture, Nagito's cheer becomes much more genuine.] She wanted to bring despair to her classmates, as well, and targeted them specifically. But Naegi-sama, he resisted her! He rallied his classmates even as some of them fell to her despair and revealed himself to be the Super High School Level Hope. He completely overpowered her despair and destroyed her utterly. The only reason the world can recover is because of Naegi-sama, you see?