Mechanical Snowman said:
My point was, that Luna attempted mass genocide and that should not be forgotten just because she says "my bad" and flutters her eyelashes.
Luna was driven to develop a shell-like alternate persona (the mask or façade presented to satisfy the demands of the situation or the environment and not representing the inner personality of the individual) complete with a new name, new appearance, new powers, everything, by a bunch of selfish, uncaring, unfeeling, ungrateful ponies (that apparently includes Celestia).
Nightmare Moon attempted to make them all to pay attention to her, possibly for as little as one day, maybe it was even a matter of minutes or hours, and for that rule breaking and not changing it back immediately at Celestia's say-so, she was beaten up by her sister and imprisoned for 1000 years (at least, that's what
Celestia's history books say, the real story might be less favorable towards Celestia). The Nightmare Moon persona would have obviously helped Luna deal with the imprisonment. That sad moon picture isn't accurate.
Then some prophecy (Whose prophecy? Did Celestia write it?) predicts the exact time of her escape, an escape performed with the help of the stars, which seems to implicate Twilight somehow. Perhaps by being convinced by Celestia that Nightmare Moon would appear (and that she would destroy the world, somehow the prophecy got that little part wrong), Twilight's need to be right made her unconsciously use her magic to draw Nightmare Moon towards her, providing a weakness in the trap that Nightmare Moon was able to capitalize on. That might also explain how Twilight was able to nail down the exact place of her appearance as well as the time.
So, after using her omniscience to push Twilight the Chosen One into "random" encounters with all the other Chosen Ones and Nightmare Moon, Celestia ran off and hid somewhere and let the ponies think she had been harmed somehow by Nightmare Moon (notice how Pinkie-Pie straight-up says that Celestia's only hiding), instead of fighting her own battles or just saying "Hey ponies, guess what? My sister just came back from the moon! Lets throw a big party to welcome her back!"
Then after doing little more than accidentally crashing a party and being glad that even
one pony there knows who she is, the stupid ponies try to arrest Nightmare Moon for no apparent reason (Celestia ditched her own party? Someone go arrest Luna for it), so she ignores them all and goes home only to find that her house is wrecked because Celestia got bored with it. Celestia apparently got bored with her battery-powered super weapon too, because she just left it sitting there in the hall. The "heroes" don't know what "go away" means, so they barge in and beat up Nightmare Moon and make Luna cry, just in time for Celestia to swoop in and play good-cop.