Why

Nagisa used to be the top bad boy of the school, until his Dad died.
His Dad was the nicest, most generous person, and Nagisa always looked up to him.

Karma hated Nagisa. He should be the top bad boy, until suddenly one day he was.

Confused by how he had such reputation all the sudden he watched Nagisa in class and saw a tear roll down his face. He decided to give up his hatred for him and let him be.

Then both of them where placed in the End Class.