I like to think that when Pitch got transformed from Kozmotis Pitchiner to the Nightmare King by the Fearlings, it was not an instant change, but a slightly more gradual process, lasting around a few days.
Just imagine that happening. Pitch would be slowly watching himself turn into everything he has fought against and everything he sought to rid from the world. He would be shocked at the thoughts of darkness and pain entering his mind. Horrified that he gets pleasure from those thoughts. Trying to hang on to what little remains of his sanity, but powerless to stop the darkness within him from spilling out.
Using what little control he has over himself, he goes begging his friends to kill him. But they refuse as they only know Pitch as a heroic general, not knowing what he will become. He then tries to end his own life. But strengthened by the hundreds of Fearlings that are taking over his soul, he fails.
He desperately tries to cling on to the memories of his daughter before they vanish forever from his mind. He goes through each and every moment of happiness he’s ever had with his daughter: reading her a bedtime story, going on long walks with her, carrying her on his back, knowing that he would never see her again. But he’s too afraid to go try and meet her one last time in case he loses control of himself and hurts her, or even kills her.
Not wanting his daughter to know what he has become, he writes a note pleading to whoever who finds it to tell his daughter that he has died valiantly in battle. He looks over at his locket one his time, tears running down his face. In his last act as Kozmotis Pitchener, his last act un-manipulated by the Fearlings, he presses his lips to it and says farewell to his daughter, the daughter he will never see again:
Goodbye my darling, please forgive me for whatever I do when I don’t remember you. Be strong and take care of mommy for me. I love you.
And then -
Kozmotis Pitchiner is dead. The Nightmare King is born.
*Fanart from :http://joscomie.deviantart.com/art/RotG-locket-340703451*