The very first antagonist, alias of princess Luna. But what really is it? Supposed origin of NMM are Luna's negative feelings such as jealousy, anger etc. Does it mean that NMM is the true Luna? Those feelings and thoughts came from the inside which means that's who she truly is.

Now, the question is what did elements of harmony do? They "purified" her. Removed "dark" thoughts. But is forcefull removal of thoughts a good thing? Is it right to make a person behave like you want them to? Is such brainwashing justified?

I agree with 5431138, to a point. I believe Nightmare Moon was a corruption of Luna, but not necessarily from an outside source. Rather, it is also possible that she gave in to destructive and irrational emotions, ultimately corrupting herself. As for whether it is who she truly is, based on this hypothesis, I'm afraid the best answer is yes and no. It is an aspect of herself that she permitted to gain dominance over the whole.

Now, regarding the Elements of Harmony and her purification/brainwashing/whatever you wish to call it: I suggest that the remaining aspects of her personality, rationale and the love she still held for both her sister and the kingdom they once shared, had desired the removal of this corruption, but were unable to accomplish this due to its deep-seated nature and required outside aid, not unlike someone resorting to hypnotherapy to quit smoking or lose weight (though on a much larger scale).

My thoughts; take 'em as you will

5431127 In my writings, I have not been fair to the Elements, but my actual beliefs differ from my writings so.

The Elements work like sucker punch. It's not brainwashing. Brainwashing is the removal of other thoughts. The Elements are the forceful input of other thoughts, or this case, the forceful input of Harmony. Just like a sucker punch, which is the forceful input of one's fist into one's face.

Twilight's Sparkle actually brings up another point about NMM. According to the comics, Nightmare Moon is not a corruption of Luna's mind, but a parasite that has latched onto Luna's conscious. It is such an effective parasite that it worked on Rarity too. Now, this can be debated in the realms of canon for MLP, but it can't be dismissed because the show has never fully disproved it (though I hope they do). In fact, one could argue the darkness that came from the moon in the flashback in Princess Twilight is evidence for the comic's canon.

Now, with the other theory that NNM and Luna are indeed the same without external causes, I believe it's hard to condemn the Elements still. Nightmare Moon is a force of thoughtless revenge. She wants to end EVERYONE's happiness for her own gain. Perhaps her reasons were justified at one point, but an end goal as extreme as that only makes her a zealot. With my headcanon that she was overcome by Harmony and broke the Nightmare Moon persona because of it makes the Elements seem harmless to a point. With your headcanon, they seem more vile, but only to a point. How in this sense is the brainwashing bad? It didn't take her freedom of thought away from her, only cured her of her truly evil thinking. It turned a bad mare good without making the mare a proudless husk. She wasn't filled with propaganda, she was turned to the righteous path. It's not the shutting of doors of freedom, it's actually the opening of other ways to think. That's the complete opposite of brainwashing in every sense.

Is it right to make a person behave like you want them to?

This never happened with Luna. I doubt the Elements or Celestia would want her to create her own nightmares to torture herself. I doubt the Elements or Celestia would want her to continually have temptations to return. Luna didn't become some perfect princess, she became Luna. Nothing changed in her but what she desired. She desired eternal night, now she desires admiration. I mean, look at Luna Eclipsed. She was so disharmonious in that episode that I would believe the Elements were off completely. It's her current stage in life which makes her so popular. She's not a brainwashed husk, she's a pony striving to be loved and striving to be the best pony she can be. But just like in real life, it's hard.

5431127 Negative thoughts can make one to behave differently, especially if they're bottled up and given an alicorn's apparently long lifespan, if not outright immortality, could mean that Princess Luna might have been harboring such thoughts for several generations(never really defined in canon)before they reached the tipping point and created the identity of Nightmare Moon so Nightmare could be considered a split personality that took over.

As a split personality, Nightmare Moon could be considered a separate entity and yet still a part of Luna so your theory of Nightmare being Luna's true self still holds some water. Based on this theory, when the Elements hit her, Nightmare Moon might not have been purged from Luna but was sealed away back into Luna's subconscious.

As for the other questions:
Is forceful removal of thoughts a good thing?
Forceful removal is undeniably wrong since it violates an individual's free will. If the thoughts are harmful to said individual and others, there are alternatives. Though in Luna's case, it was a last resort considering her madness as Nightmare Moon and thus a good thing due to the circumstances at the time.

Is it right to make a person behave like you want them to?
If you ask a parent this of their child and they'll most probably tell you that they don't want to force them to become something they don't want and yet they still influence their child's behavior in different ways that steers them towards a goal that the parents would approve of though this might make the child push them away as they grow older.

Is it right? If it's done with good intentions, yes but as they say, the path to hell is paved with good intentions.

Is such brainwashing justified?
This question is a trickier since just like the first answer, it depends on the circumstance and situation though it is still morally wrong such as the time when Twilight attempted to brainwash Discord with a spell. This shows that Equestria might have used the spell to reform villains or criminals in the past, implying that brainwashing might have been justified at some point in Equestria's history.

5431138
If it was an outside influence then it must have a source and if it can corrupt a powerful alicorn then it must be very powerful itself, however nopony seems to care about the source, the means or goals of this corruption. Also this corruption happened only once in all of pony history which seems oddly convenient.

5431171
But giving in to your emotions is what makes you you, and Luna who strongly loved her sister and kingdom simply isn't the real Luna anymore because you from 2 years ago isn't the same as you now. Actions and experiences is what makes us us . You can't just rewrite someone with their past self and say that that's how it should be from now on.

I feel like it comes down to person vs society. If you are deemed evil by the society then you must be made good or isolated. The question now is how many normal things and behaviours will ponies call evil now and in the future. Is eradication of all "offending" things a good idea?

5431209 Therein lies the greatest problem of this debate: when boiled down, "Right" and "Wrong" are subjective, sometimes highly so (consider that there are people in our world who believe it is "Right" to murder those who disagree with their beliefs)

It's like a story idea. In a totalitarian Equestria any deviation from accepted social behaviour is noticed and the person's mind is rewritten/influenced/harmonised (?) by the elements of harmony to fit the society. Kind of like a dystopia(utopia?) where everyone tries to act "normal".

I feel like this is appropriate

5431227 Yet, what if somepony were able to resist the procedure?
...
Grr... too many ideas, not enough time

5431227
5431240
Starlight Glimmer's town(is there a canon name for it?) feels like what you're talking about but on a smaller scale and less oppressive(relatively speaking).
Ponies were initially all for giving up their cutie marks and their talents, believing Starlight's words over anyone else's and any deviation was immediately 'corrected'. While anypony who outright attempts to be different were sent to be reeducated until they fall back in line. Though as proven by Night Glider, Sugar Belle, Party Favor and Double Diamond, they broke out of the brainwashing.

I'd imagine that if the Mane 6 had not intervened, Starlight would have spread her influence to other nearby settlements and as their number grows. Starlight would become more oppressive in her approach and the corrections and reeducation become more heavy handed, or heavy-hooved in this case.

5431217 Well of course, this is simply human instinct. We go along with each other much like a herd of cattle. In real life, if one cow is like "screw the herd" and runs off on its lonesome, it usually ends in tragedy for that cow. The other cows see this and use it as a reason to not break away from the herd.

Is there something wrong with the herd instinct? Sometimes. It can lead people to restrict their knowledge of the world and remain "safe" within the walls of their own echo chamber. But if we didn't have this herd instinct in society, would the alternative be better?

I'm gonna say "no." If people can't even agree on what's "good" and what's "bad" then you are never going to have any sort of peace. Luna probably had good reasons to defy Celestia. She thought her "good" night would make ponies happy, whether THEY liked it or NOT. Now, the other ponies thought her evil because they used reason. An eternal night would not be "good" for crops as they need a sun to grow. Also, what ponies can't SEE, they don't TRUST just like humans. Put a human in a pitch black room full of random chairs, he is gonna stub his toe and then ask why he is doing this. He is in PAIN, so he justifies being forced into that situation as "bad." The guy watching him with night vision goggles doesn't see what's wrong with the situation and thinks the darkness is "good" because it doesn't affect his eyesight. As the ponies are forced into the "bad" situation of eternal darkness, Luna will be like "stop complaining. It's good because I say it is."

So, basically, people need to decide on what's best for a society as a whole. Murder is not "good" as it takes away the victim's right to live. It won't help society if people are constantly dying and unable to help progress humanity. Stealing is "bad" as it steps on a person's right to own materials. If there is no laws or general recognition of morals, society decays into anarchy, though the argument of anarchy can be for another day.

But here is the true question.

Is eradication of all "offending" things a good idea?

In some ways, the title of this group shows that WE are trying to do this. What is an "offending" idea to us? Well, one without LOGIC or REASON. We are trying to eradicate these ideas by using logic and reason. Stupid ideas offend us, but should we stop trying to end stupid ideas?

I say this is an endless process that will never be achieved. Is it a good idea? I say yes in the circumstance that we are always trying to progress humanity. However, we shall never agree on the same things that offend us. To some, protecting unborn babies is "offensive" and to others, having easy access to abortions is "offending." One would argue ending pro-life people would benefit society. Another would say giving an unborn baby the right to live would benefit society. Both sides are trying to eradicate offensive ideas.

If we settle to having offensive ideas in our life and cease to try and improve humanity, I believe our society would decay.

5431289 Here's where it goes even grayer, and somewhat recaps my previous point: while I agree that striving to improve humanity as a whole is a worthwhile effort, the problem lies in the fact that humanity as a whole cannot agree on what is or is not an improvement.

For example, let's say Hasbro made the Lyra-Bon ship canon. Some would applaud them for taking a "progressive" step and embracing a wider world-view. Others would be enraged and refuse to let their children watch the show, possibly even threatening legal action against Hasbro (as happened with poor Derpy) or burning Rainbow Dash plushies in effigy. I strongly believe that even we, the purveyors of Logic and Reason, would be split on whether or not this was a good idea.

In a nutshell: "if you try to please everybody, somebody won't like it". As for improving humanity from a moral perspective (which itself is subjective), it may be among the best of intentions, but for myself, I feel it wiser to improve myself first; if and when I achieve perfection, then I'll worry about the rest of the species.

Again, just my thoughts; take 'em as you will... with a final quote:

Don't try to be a great man; just be a man. Let history make its own decision.