spoiler The thing about Cozy Glow

[MLPFanatic34 3,234]

No and I simply mean what darkqiviut was saying, Cozy Glow's crime at its core is the same as Tirek's crime. 

[Disky Bolt 103]

I see the act of sending her to Tartarus not a keen act is not because of her young age,but the fact that how she value teamworks,and the possibility of teaming up the notorious and hazardous villains there.

[Dreamstar Moonlight 44]

On 10/7/2018 at 10:06 AM, Steve Piranha said:

Those are just assumptions. No one knows what’s behind her life, but facts are, she manipulated and endangered several ponies for the sole purpose to conquer Equestria. Child or not, she’s one of the nation’s greatest threats it has ever faced

Here, I found this short comic that delves into cozy's backstory (while staying true to the canon). It's a fantastic read, and makes the whole thing a lot clearer, albeit darker.

https://www.deviantart.com/lunarcakez/gallery/72531181/a-dim-glow-mlp-comic