I’m honestly glad this is how Cozy’s story ends (for now), not just because they deserves it, but watching the fanbase screech and scream about how wrong it is is pure entertainment in and of itself. Not everyone CAN be redeemed, and the shit they did is pretty unforgivable.

I’d take this over whatever the show was trying to do.

Kids can be either good or bad, but they are never ‘kill my enemies and usurp the throne’ levels of bad,  
without some really awful influence from parents, guardians, or whoever they look up to.
 
Like all MLP ponies, fillies are even more inclined to be nice and sweet when happy, and frightened and defensive (running, hiding) when upset.
 
Cozy Glow is nothing like any other pony child in the show.  
And with no explanation for why the hell she is so damn awful and evil (thanks writers!),  
This one is the closest to making any sense.

Cozy Glow did nothing wrong.

His goal is to get angry pony parents forming a twitter mob spamming Twilight with #FreeCozy and #DownWithTyrantTwilight. 🙂

@Background Pony #369B
 
Or act like she isn’t in possession of moral agency due to being precocious and fully aware of what she’s doing.

I’d read that fan fiction.
 
More than Cozy’s fate I was disappointed that we never got to see the real Grogar.

“She is a monster in the form of a little girl.”

She’s a loser! XD

No sir I don’t like it.

Meanwhile some people are trying to downplay the severity of her crimes.

@Background Pony #310B  
Where is that being justified here? The OP is assuming she wasn’t, and suggesting alternative.

Expectedly, people are now trying to justify an outright execution of a child.