But as time went on, the younger sister became resentful. The ponies relished and played in the day her elder sister brought forth, but shunned and slept through her beautiful night. One fateful day, the younger unicorn refused to lower the moon to make way for the dawn. The elder sister tried to reason with her, but the bitterness in the young one's heart had transformed her into a wicked mare of darkness: Night Mare Moon. She vowed that she would shroud the land in eternal night.
That's a transcript of the opening narration. I see only one thing in there that might,
might hint at a dark side, but even then it's a reeeeeeal big stretch, unless you're chooising to take it super-literally.
And as for what happened later, again, Luna could have just been acting that she had been 'purged' or whatever. She could have realized, "oh fuck, I'm powerless, surrounded by ponies with the weapon that sent me to the moon, and my godess sister is here now to."
Her options were lie through her teeth, or take a trip back to the moon
if she was lucky - which would you choose?