So, If you read the first chapter of my theories book, Than you know I thought Luna got so fed up with Celestia,Sombra, and Snowdrop's death...Just read the below.
What if Luna was banished, Yes she was jealous of Celestia, what if Snowdrop's death added to that? And King Sombra betraying her was the last straw
Let's see, In one of the last lines of the story of Snowdrop: She was the only one who truly appreciated my night. Ooo! That made her come closer to becoming Nightmare Moon. Then she was in love with Sombra (I believe two theories of him :p ) He may of dabbled in dark magic as the king of the Crystal Empire. Or he may of wanted to become a unicorn. Anyway, he was courptted (I can't spell the dang word) and Luna felt betrayed. They battled. Blah blah. Luna feels like it's Celestia's fault. She turns into Nightmare moon.