The reason why Princess Luna turned into Nightmare Moon has been alluded to, theorized, and dramatized. But all of the reasons why have been grandiose and very often external in nature. But all that is really needed to turn someone from a being of good to a being of darkness is but a simple whisper and time. A whisper given by a force of hatred and fear.

Picture is used with permission and is by Warepwn3 on deviantart. Found here

Also if you want a more epic and animated view of basically what I would have loved this story to be go here.
https://www.youtube.com/watch?v=vL4q7BBzanI