Talk:Starlight Glimmer/@comment-1276314-20160329195457/@comment-1031724-20160404232433

We know Luna began feeling jealous, bitter, and resentful on her own. My theory goes that, Sombra could sense this when Luna and Celestia banished him, so he cast a curse to magnify that bitterness so it would drive her to become Nightmare Moon and overthrow Celestia. (part of it went that Nightmare Moon was an entity of her own possessing Luna, but later episodes proved that to be wrong)

It was hinted by the creators that an outside force was involved in Luna's transformation. Luna herself is likely unaware of this.