I kind of figure this is what they'll end up doing with Eclipsa. Instead of a "Hahaha, Star I was evil all along, you fool!!" It's more that, she's not really evil per-se, just more selfish and probably a sociopath.

She does what she wants, when she wants without any regard as to how it'll effect others. Like abandoning her entire kingdom to fuck a monster, or creating spells that ignore basic morals of others. Being able to spy on others for one, or killing them off completely as examples.

She's never done anything truly evil because she's never needed to before to get what she wants, as her desires have been pretty simple up until now. Like candy, or flowers, or monster sex.

Moon: Why the dark magic?! What are you planning to do?
Eclipsa: Whatever I want! Not everything dark has to play the big villain, queenie: I ran away from my arranged marriage with a monster. I'm not threatening your kingdom, just looking out for number one.
Moon: Star was right. You aren't evil, you're just selfish!