Yes. She decided that if they wouldn't see the beauty of her night on their own, she would force them to, by any means necessary. Her sister tried to make her see reason, but it was far too late. Her emotions had transformed her into a wicked mare of darkness... Nightmare Moon.

[Nightmare Moon = Heartless?]

In her anger and jealousy, Nightmare Moon declared that the ponies would never see the sun again... that the night would last forever.