@Mr. Horrible  
Needs a bit more info/execution but this is interesting.  
After all, NMM isn’t doing it “for evulz” but she just wanted her hard-work to be loved as much as Celestia’s and no longer be in her shadow. Pretty much she’s consumed by negative emotions and she is basically lashing out, telling her she’s screwing up and being just as bad as someone also bad(realistically like Sombra, who literally ruled with fear), the realization would snap her out of it.  
I prefer something like this rather than forcible purifying her with a rainbow laser, she reforms/resolves her issues out of her free will instead of just essentially by force via purification.

 
NM: False, what fascination we have there.

it like she relaps.

Nightmare Moon: That’s it. I’m done. I’M DONE! STICK A FORK IN ME, I’M DONE! GIMME THE MIC!
 

X.X

I really loved the twist at the end where it’s not some magical turnaround for Blueblood to save the day, but that Blueblood is such an immeasurably terrible person that the Mane 6 would rather join forces with Nightmare Moon just to get rid of him.
 
And that a being made of pure hatred, jealousy, and malice like Nightmare Moon could be made to see the error of their ways just by being compared to someone as awful as Blueblood. Pretty fantastic.

I’M A PRINCESS AAAAAAAAGH! and I deserve the respect according that title
 
AAAAAAAAAAAAAAAAGH!