So, having had time to read some of the MLP fan-fiction of late, I wanted to address something that's been bugging me for a long time now. See, in many of the adventure/action stories I've read that involve the mane cast, the fanfics either turn Luna back into Nightmare Moon or explain her descent into darkness as the result of mystical evil artifact/mysterious force/plot device X to give whatever new thread the author has conjured up has some tenuous basis in canon. This is a major point of contention with me for several reasons (negates Luna's redemption, relegates Nightmare moon to mindless evil villain #4, etc) but also because, well...

I don't think Princess Luna was ever actually evil.

Putting aside the fact that one of Equestria's ruling monarchs going evil would surely be written in pretty much any history book to have been released during the time period (not relegated to a single book) think about the events of "Friendship is Magic" part 2. Despite supposedly being more powerful than Princess Celestia (enough to have banished her or whatever), does she ever really confront the team? No, not really. She puts a few half-plotted schemes in their way, none of which are really life threatening or anything. Even when confronting Twilight Sparkle, she never overtly attacks... about the most evil thing she does is break the stone elements (which, having helped wield the elements against Discord, she should know wouldn't do anything). If she were really trying to stop them from interfering, she could just have killed them outright.

Contrast that with Discord. Sure, Discord never overtly attacks the ponies either, but he's not trying to physically stop them the way Nightmare Moon was. Instead, he's toying with them, torturing and twisting their love and friendship against them. He brings his full power and cunning to bear, and in doing so is a much more credible threat.

But if Nightmare Moon wasn't trying to really hurt the ponies, what then? Well, let's look at what we know.

-Instead of being a major historical fact, Luna's banishment is relegated to a single mention in an obscure book that just happens to be in the library Twilight later lives in.

-Everyone kind of blows Twilight off when she talks about the "Mare in the Moon" legend, but there is an audible gasp from the assembled ponies when she reveals Luna as "Nightmare Moon," the monster of Nightmare Night. In other words, the ponies do know "Nightmare Moon" as evil, but they don't really know why.

-When trying to "stop" the Mane 6, Nightmare Moon makes a pretty pathetic effort. However, each and every one of of her "traps" is mysteriously designed to bring out each of the elements of Harmony, including separating Twilight from the others.

-Luna smashes the elements of Harmony, despite the fact that, having wielded them herself, she'd know wouldn't destroy the elements themselves.

And finally, the last and most important bit of information

-Princess Celestia is a total troll.

It is my theory (and admittedly this is my own personal viewpoint) that Luna's "banishment" and return was a ruse. Princess Celestia likely knew that the elements would come to rest in somepony else's hooves, and that they would be needed again (either because of Discord or for yet another threat). By crafting the fictitious villain "Nightmare Moon," she and her sister were able to prepare the girls in a relatively controlled setting.