Villains can be forgiven and reformed if they value friendship, love, peace, abundance and/or prosperity as the top priorities.  
(These are long-term satisfactions, it is like getting fruits from a slow growing plant, but it gives you for long time)
 
They’ll fail to reform if they value grudge satisfaction, winning and/or control manipulating over what’s good mentioned.  
(Yet, these are short term satisfying but never in long-term, took sometime for people to understand this, it is like enjoy hunting rather than farming)
 
The priorities of what you values in your mind chooses the side you are in for you.  
In conclusion, Starlight values things in life differently from Cozy, and Starlight already knows what are long-term happiness is like, from that she SHOWED HER CHILDHOOD PROBLEM, she wanted to have friends like how she spent with Sunburst back very desperately, so Twilight feels sorry for her and understands her, while Cozy barely understands that, and don’t even reveal her problems to any others, so nobody understands her, since she kept all her mental problems by herself.
 
Ages doesn’t matter, if you are young and do things wrong, it is even MORE WRONG to use your young age as an excuse. In real life, people call bad children “brats” and chase them out from their territory after all.

@tehwatever  
I see

@Background Pony #E661  
Starlight has a lot of moral doubts, but she is really a social pony and she has a lot of remorse.

@Jisow777  
Psychopath: persistent antisocial behavior, impaired empathy and remorse, and bold, disinhibited, and egotistical traits.  
Sociopathy: Same as above, but lacking the ability to understand right from wrong in a moral sense.

Starlight obtained friendship and learned a better way after struggling with the desire for personal revenge against Twilight (I.E. not intending to harm anyone else).  
Cozy already had friendship, but instead, stabbed EVERYONE in the back (even those who trusted her like Twilight and Starlight) in pursuit of her goal, which was literally to rule Equestria for no reason other than a self driven lust for power and control. This is a defining characteristic of a psychopath, by the way.

@FurriesandFlurries  
Cozy definitely has some of the symptoms of a psychopath wile Starlight a sociopath

Here’s the thing, Starlight was clearly a mental who had good intentions along, but handled them in the wrong way and was furious of all she had worked on be taken away from her. So she decided to enact revenge on the person who caused it so she could remain living in her own crazy beliefs. But as her opponent became more and more relentless, she became to break down and grow frustrated with the fact that someone was giving her a reality check. Plus, her Sunburst backstory has a sick twist in it as the way she tells it, her foalhood friend who was the only pony she felt happy with, straight up ditched her and never gave her a second glance afterwards. You gotta wonder, did Sunburst even realize how much he meant to Starlight? Also, we don’t know what Starlight went through later on in her life, but I think that the most reasonable answer is that she was incredible insecure about herself and her distain of cutie marks grew from seeing how it changed people to the one where she couldn’t stand even looking at them and wanted to be rid of them. But when she was faced with the choice to either rip apart the scroll or change her life for the better, she made the call, not through being stripped of her powers, but her own conscious. Plus, she was willing to accept her punishment, but Twilight knew now that Starlight had come back into the light, she could help Starlight restart her life for the better.
 
Now, Cozy Glow, on the other hand, has downright selfish motives that would drain all of Equestria’s magic and give her the power to create a dystopian society. Her intentions were evil and solely evil, and that’s why she got what came to her. Plus, did anyone catch that she was associated with one of the most threatening beings Equestria had ever known, that being Tirek.

@Hakirayleigh  
Who’s to say all those timelines were starlights fault. Ever heard of multiverse theory. Maybe it just sent Twilight to a series of different universes. It would explain why Twilight and Spike don’t seem to change.

@tehwatever  
Yeah, some people (specially her detractors) say she has the least evil intentions of all villains, but her destruction it was the most impressive (But I believe those multi universe were only resets). However O believe Starlight designed very well her plan and don’t let the remane5 doesn’t rise uo Twilight’s mood.

@tehwatever  
Yeah I kind of suspected that the elements of harmony play a art in reforming villains. It usually takes at least to hits. Luna twice, Discord twice, Tirek just once and Sunset once. Chrysalis is probably the only one who hasn’t gotten blasted and she still hasn’t been reformed. Also Sombra but he’s dead as far as the show is concerned.
 
I kind of want to See Cozy team up with Tirek. For an Ironic turn of events have her betray Tirek and steal his powers. I think that set up would lead to Tirek being reformed though and I’m not sure I want Tirek to be reformed. Chrysalis maybe but not Tirek. I think Chrysalis is more redeemable then Tirek. There’s a chance she may have just been doing what she was doing for her hive and just hated being undermined.

@pelcogo  
Starlight being reformed at the end of S5 is a lot more accurate than saying she was redeemed so I agree with you. In the scale of evil, Starlight was probably the least evil out of all the villains they fought so far, Sunset included. You made a good point in that Starlight had some good in her that Twilight could appeal to, that’s why she could convince her to stop on her own–without having to blast her face with Rainbow Magic (which I think works like a drug that calms you down)
 
It’s to be seen if Cozy Glow can’t be reasoned with. S9 better bring Cozy Glow back to school as a reformed villain.

Starlight couldn’t have been that bad if she was able to be reasoned with. The difference between Starlight and Cozy Glow was that Twilight was able to reason with Starlight instead of thwarting her plans. Reforming Cozy was not an option. She almost literally made a deal with the devil. At Least everything Starlight did could be undone so it’s easier to sweep her past sins under the rug.

@tehwatever  
Well she was reformed at the end of season 5. SHe was willing to accept her punishment but Twilight knew she would do more good as an apprentice then behind bars and while there has been hiccups along the way it proved to be a good call at the end.

Way tl:dr

@tehwatever  

 
Ook

@tehwatever  
I consider her official redemption was in the season6 finale.

@tehwatever  
THANK YOU

@Starswirl  
Keep throwing around the word redemption (even with quotations) like that and it would lose all its meaning, just like how Rainbow Dash abuses the word “Awesome” and now it doesn’t carry the same weight as it should anymore.
 
Starlight didn’t get redeemed at all in Season 5 finale. MAYBE she got hers in Season 6 finale, or heck maybe on Season 7 finale. To be REDEEMED means to have done something good that makes her past deeds more or less “even.” To be redeemed means to be “debt free.” She didn’t even start paying off her debt until after she became Twilight’s full time student!
 
Starlight, at the end of the goddamn S5 finale, was FORGIVEN. Major difference! Twilight was just a super nice pony who gave Starlight a chance and gave all kinds of gifts that would jump start her journey towards earning her redemption. I will never stop saying this: Starlight, was forgiven, NOT redeemed.
 
Her Redemption Arc ONLY BEGAN by the start of Season 6! You guys, I think you’re not being fair, here.

@Background Pony #E661  
Because that’s the only thing Starlight did. Right.
 
@CaptainGrumpy  
That, and Starlight’s “redemption” sucked.

@Background Pony #E661  
yeah I don’t know how people can defend Cozy Glow, is it only because she’s a “child”

“I’m going to alter time to take revenge on someone!”  
Well, I mean it is kinda a shitty thing to do, and you could potentially screw up way more than you intend to, you clearly don’t want to hurt anybody else, even though you might.  
“I’m going to wipe the world of all magic, so I can backstab and imprison someone I know personally and have no reason to harm, along with their friends, so I can seize their position. In addition, I will also imprison/banish anyone who may stand in my way, manipulate others into aiding my cause without their knowledge, work with a hostile entity, and in the event that I am defeated and offered a second chance, I will refuse and attempt to flee and resume my original goal.”  
YEAH OK YOU GO DO ALL THAT. HAVE FUN IN HELL.

The infinity war that is this comment section.

@tehwatever  
Genervt and I were talking about Sunset, not Starlight.
 
And her line just before placing the crown on her head ( “At last. More power than I could ever imagine.” ) paints a pretty clear picture of what Sunset had in mind even before putting on that crown.

Starlight’s probably got some kind of guilt and/or self-doubt after Cozy’s deceit.  
Not being able to convince Chrysalis to be her friend is one thing,  
not being able to convince a little kid to embrace friendship is another.
 
Presuming of course Cozy really is a little kid,  
and not some weird demon that’s old pals with Tirek or whatever.  
And if she really is simply a little messed up evil kid,  
that’s still an astonishing move for a family show to portray an apparently irredeemable child.  
Makes the message muddled.

@Background Pony #64D7  
Until she put on the crown she did nothing that could be considered unforgivable.  
She was “just” a bully and a thief.  
No Moral Event Horizon.