This was a hell of a lot better than I expected.

1. Everything Twilight did to desperately try and fix everything makes so much sense. She tries to fight Starlight mentally and physically, but SG was so hellbent on revenge and the pain of her past, she'd do everything she can to stop it.
2. Don't look at the fact that SG lost her only friend as the cause. Look at how she lost him. The second he got his mark, she never saw him again. In other words, if he never got that mark, everything will be completely different. She'd still be home. Sunburst won't go off to Canterlot. They'd still know each other. No cutie mark, no memories of her past.
    
   Like that, her actions, as evil as they were, feel plausible. Without the pain of individual cutie marks, then no one would have to feel the same pain as she did. No wonder why she concocted the spell of the equality mark.
3. There are two great morals to the story.
    
   a. You can never change your past. You can only change your future. Starlight can't change what happened with her in the past, and thus she can't inflict the same pain on Twilight. But she can change how she goes about things. Restart viewing cutie marks and friendship altogether in a much more positive light and use it for good.
    
   b. One moment makes a difference. This calls back to a similar moral from Amending Fences: You might see the moment as inconsequential, but to others, it can make a really big impact for better or for worse. Twilight's decision to skip Moondancer's party helped change her for the better, but it changed MD for the worst. For Starlight, it's the same thing. Her one moment back home makes a big impact on her life, but she was so hellbent on revenge that she never stopped to think about how the one moment from Rainbow Dash changed everyone's lives around them. It'd make everyone's lives miserable, even herself.
4. The way it ended may be a little cheesy, but it's the good kind. Starlight assumes that by changing the way the Mane Six would meet, Starlight won't have to have her village collapse. But she never foresaw how it'd hurt her, either. The only way for her to actually see it is to take Starlight to an apocalyptic future and then use this and how Starlight grew to being the mare she was to try to change her for the better.
    
   What happens here is way more fulfilling than Sunset Shimmer's. Sunset's whole character was reset, and you saw her redeem herself as a completely different character. Starlight never changed her character. Instead, she was convinced, and she willingly accepted Twilight's apprenticeship. As a result, Starlight's addition to the Mane Six (now Mane Seven or Mane Eight, if you count Spike) feels more genuine.
   

I'll need another viewing to gauge its quality, but right now, I'm loving The Cutie Re-Mark.

Source: S05:E25+26 - The Cutie Re-Mark