Nightmare Moon is not a sympathetic character, and you're probably an idiot if you write her like one* · 6:35am Apr 17th, 2015
*This post assumes NMM's plan to bring eternal night would have ended the world, not simply changed the skybox texture, the moon is a physical place, and she was aware. I feel the need to say this because we've seen some fucking ridiculous things happen to the Equestrian sky, so I don't feel comfortable saying their sun and moon are similar to ours without also qualifying my statement. If your story doesn't follow these assumptions disregard this post.
Sending NMM to the moon for a thousand years can hardly be called a punishment because she got almost exactly what she wanted: To rule an empty world. We know it wouldn't have been super dark on the moon because its light shined down on Elysium, so either it is also a light source or it reflects light. She didn't get her eternal night, but she got the consequence of eternal night. I can(barely) accept her initial rage being a crime of blind passion, but she had a thousand years to calm down and contemplate her existence on the moon. She still decided making Elysium in the moon's image was a good idea. She wasn't a wounded, sympathetic bird, she was an unapologetic, omnicidal maniac.
Discord, Chrysalis, Sombra, Tirek, and Starlight Glimmer are all more sympathetic characters than NMM. Either they're demonstrably better people than NMM, or we don't know enough about them to say for sure they're unrepentant monsters. They have wiggle room for interpretation:
1. We know ponies were able to live through Discord's shenanigans, and he does moralize to Celestia about her habit of imprisoning people for inhumane amounts of time.
2. Chrysalis was trying to feed the Changelings, although that's more of an implication than something we know for sure.
3. Sombra was just a tyrant as far as we know. Hardly sympathetic, but he's not, y'know, an omnicidal maniac.
4. Tirek was a soul-raping thief, but we didn't see any indication that he sought to pull a Kefka with his power.
5. Starlight Glimmer might have truly believed her ideology was for the betterment of all ponies.
See what I mean? Some of them are almost noble, and the others at least might not be as bad as NMM.
That, or Nightmare Moon was a bloody idiot. Or insane.
Tirek is debatably worse since he sought to steal the essence of another's life without truly killing them, which one could say is equal to or even surpasses murder. For what kind of life would that be? Under the rule of one such as he.
Same for Sombra, who most likely conquested for world domination and appeared insane. What kind of life, under that kind of rule, is worth living? And when magic's involved all bets are off when it comes to rebellion as he would surely have suppressed the magic in all species, leaving only him with great power. And as a tyrant such as he, he would be paranoid beyond belief of traitors, meaning he would most likely not grant power to his royals (if he were to have any), that is of course if he was, at that point, even capable of caring or trusting another individual. Which he probably wouldn't be.
The others check out though
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Oh, please, stop being so pony-centric. It's not just about them, it's about the whole rest of the world. You're mistaking the actions of a Mao Ze Dong-type dictator for the actions of someone trying to extinct all life.
Remember, Tirek just wanted magic, he didn't say anything about becoming king.
I've always wondered why people always assume Nightmare Moon was completely conscious on the moon. She was called the mare in the moon, so I always thought she had been sealed and the thousand years just passed her by.
I mean, the effect a thousand years in that kind of environment on a mind is scary.
The question remains to how much NMM was actually a full, independent entity wholly separate from Luna that just possessed the alicorn's body and mind, or if NMM was really just the darkest, twisted part of Luna's psyche brought forth due to extreme circumstances related to potentially whole centuries of neglect from everypony on the planet including her own sister (Who everypony loved).
Or, in other words - NMM was literally nothing but the absolute worst-case scenario of Cutie Mark Failure Insanity Syndrome to ever happen.
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I kinda sidestepped that issue because the Elements put a reset on Luna. If it was just an angry, omnicidal Luna, then they reprogrammed her. It if wasn't an angry, omnicidal Luna, then they purged her.
Either way the events surrounding NMM are pretty intense, Luna gets mind-raped one way or another.
Got to agree with this, we have yet to see any other villain whose plans would have ended with anywhere near as high a body-count as NMM. I mean, maybe some creatures can survive on a tidally-locked planet (Dragons, who don't need to eat plants directly or indirectly, and could live in lava if they had too) but most life would be gone.
The only exception is Tirek, since he took away the magic of all the alicorns and unicorns, and as far as we can tell he didn't really gain all the knowledge and skill of those whose magic he stole, just the raw power, eternal day/night for various parts of the world was going to happen under Tirek as well. Of course, that wasn't actually his goal, he just didn't care.