Murderer's Guilt-Nagikae

Nagisa has constantly been battling with the guilt he has from killing Korosensei, he has been blaming himself for the past 9 years. While he has still become a reputable teacher, he still deals with the guilt. Hes managed to keep it a secret from most of his friends. I say most as Kayano found out and is now trying any method to eradicate his guilt.