They say history is written by the victors, and that was certainly the case when it came to Equestria. Cozy Glow is a difficult pony to describe. It would've been much easier for Twilight's sake if she didn't have to explain to everypony who Cozy Glow was. What's funny is, immediately following Cozy's stunt at the school of friendship, keeping Cozy's name out of the public consciousness was exactly what the princesses had in mind.
Here's the thing about the magic incident. Virtually no one in Equestria (besides those directly involved at the school) attributed the incident to Cozy Glow, because virtually no one even knew about her. In fact, the anomaly was mostly thought of as a magical mishap with no underlying cause.
Twilight Sparkle and Princess Celestia chose to keep the fact that the great calamity was the work of Cozy Glow under tight wraps. There wasn't any reason to publicly identify the pony responsible. Hardly anyone would've probably believed it anyway. But this led to some awkward situations. Cozy's anonymity to most is what allowed her to be so effective in plotting her new scheme for total domination using Grogar's Bell. Feather Flatterfly didn't know who Cozy Glow was during the Summer Sun Celebration. He didn't recognize the filly who was responsible for the great catastrophe that nearly erased all of the magic, because it was never publicized that she was to blame.
But as for the final showdown versus the Legion of Doom leading up to Twilight's coronation, there was no keeping that a secret from any creature, because practically every creature in Equestria was already there. To every creature, this was their introduction to Cozy Glow. A raging alicorn filly.
After her defeat, word about Cozy Glow was not flattering. But it was what wasn't conveyed to every creature in particular that painted a narrative not in Cozy's favor.
Any word that Cozy Glow was released by Discord, and united with Lord Tirek and Chrysalis by Discord, and tasked with retrieving Grogar's bell by Discord himself was completely non-existent among anyone but those closest to Twilight.
Every creature ran with the idea that Cozy Glow and Lord Tirek "escaped" from Tartarus, joined with Queen Chrysalis, and used Grogar's Bell that they found. They didn't question how the villains united, or why they chose Grogar's bell specifically, or how they mastered the bell's powers.
Magnolia knew Cozy Glow, so she was the only one who cared enough to ask these questions, and uncover the holes in the popular belief that the Princesses chose not to reveal.
The truth was that those in charge chose to lie by omission. The less everypony knew about this final incident, the better. Word that this was all the work of Discord, someone extremely close to the princesses, could've created a huge mistrust in Equestria's leadership. Nothing in any publication was necessarily incorrect, but it wasn't the full picture. And thus every creature was none the wiser.
But with these missing details, and an incomplete story, it made defending Cozy Glow extremely difficult. It was impossible for anypony to see her as anything but insane, and always seeking mischief. A filly who chose the path of villainy, and ultimately got what she deserved.
But Maggie had assumed there was more to her. Cozy might've once had a good side, and at one stage in her life only wanted to win over the friendship of her friend in school.
When she saw how terrified Cozy Glow looked as a statue, she saw the innocent Cozy who kissed her.
But then there was the anomaly that was the Cozy Glow that nearly broke Maggie and her mother. The Cozy Glow who was so laser focused on getting to Twilight's school, and little else. The Cozy that eventually came so close to ruling Equestria twice.
What in Cozy Glow changed after their expulsion? Was she truly innocent, or had her true ambitions emerged after her injuries? Was innocent school Cozy Glow the real anomaly?
But with no way for her to undo the curse, and talk to Cozy Glow herself, it was impossible to do anything but guess.
Not that any of these factors excused what Cozy Glow did, but everypony deserved to have the extra context. Knowing that Discord was responsible for setting up the Legion of Doom to attack, might've changed some minds, but maybe it wouldn't've. Maybe if Magnolia could talk to Cozy Glow after she was shown humility, and ask her what she wanted, maybe a part of her that was once thought to be lost would've come back to light. But that was just hopeful thinking.
Evil or not, Cozy Glow was a pony, and one Magnolia Valentina used to know. So, Maggie personally couldn't see this incident as anything but a tragedy regardless of Cozy's moral compass. It was tragic that a filly would be conditioned to carry out such an incredible act of treason for the sake of power. But Maggie wondered what conditioned her to be this way. That was the real mystery that Maggie had nightmares about.
The reason Maggie cared so much about finding out the truth about Cozy Glow, was because Cozy Glow reminded her a lot of herself. And if a pegasus like Cozy could go insane and do crazy evil things, then what was stopping a powerful unicorn like Maggie from doing something similar? Or any other filly for that matter? But without uncovering her true nature, then how would anyone know what to look for in order to prevent another incident like this from happening again?
Cozy Glow was defeated, but it didn't seem like anyone learned anything from her defeat. Twilight Sparkle merely learned that fillies can be evil, but didn't learn the reason why fillies could be evil.
Magnolia had made up her mind. As the years went on, the mare continued her training in the royal guard, and later in her life would eventually serve full time. She used to look up to Twilight Sparkle as the ultimate bringer of friendship, and vowed to protect the land she commanded. But after learning of Cozy's fate, she had a different motivation. She may not have been able to help Cozy Glow, but she could still help other fillies in Equestria. If a filly like Cozy could rise up, surely it could happen again some day. But it wouldn't happen under Maggie's watch. The sole reason she stuck with the royal guard was to make sure that no filly ever ended up like Cozy Glow ever again. And should a pony like Cozy Glow ever rise again, may Celestia have mercy on their soul...
The End.