I have a theory on why Starlight Glimmer is a villain and why she's so obsessed with taking off Cutie Marks and making ponies the same. When she was little, Starlight remained a blank flank for long time (like the CMC), and thus, she was bullied by other fillies who already had had their Cutie Marks (like Diamond Tiara and Silver Spoon). One day, the bullies pissed her off really hard, and she fired a beam at one of the bullies, taking off his Cutie Mark, and earning her own Cutie Mark in the process. And since that day, Starlight has been obsessed with making everypony the same. No one is special. No one is different. No one can bully others just because they don't have Cutie Marks, because no one has a Cutie Mark.
What do you guys think?

4724239 Idk(maybe because i have a story that has a diffrent idea why XD )

It's plausible.

Maybe Starlight Glimmer is Terminator Sweetie Belle from the not-so-far future, coming back to put an end to her own torment and change the dark future of Nightmare Tiara.

Could be a lot of explanations, really.

4724789

Maybe Starlight Glimmer is Terminator Sweetie Belle from the not-so-far future, coming back to put an end to her own torment and change the dark future of Nightmare Tiara.

4724239 while I have a personal beliefs that there could be some behind Starlight equality beliefs maybe he is acting as a teacher to her.

4724239
The equalizer spell is not some mutant special ability that she had since young, it's a spell she studied and developed when she's an adult mage, for the expressed purpose of removing cutie marks. She implied as much when Twilight blocked it with her shield.