I think the problem here is, we've all assumed he was a villain in the first place.
He's modeled after Q, from Star Trek: Next Gen. He's portrayed from the start as an antagonist, casting himself as judge and jury of the human race, telling Picard they're a race of barbarians that oughtn't to be set loose on the galaxy.
But then, for all his power, he doesn't really stop 'em, does he? Indeed, in the end, everything he does seems almost... helpful.
Take, for instance, the first episode involving the Borg. Q throws the enterprise across the galaxy, where they find themselves face to face with their greatest enemy yet. A race of cyborgs that absorb all that they find into their collective.
Many people die in this fiasco.
And yet... they discovered that the Borg were out there. When the invasion came, humanity WASN'T crushed and absorbed. A few deaths seems a cheap price to pay for that.
When Discord first took everything over... Tirek wasn't a thing yet, Sombra wasn't born, Nightmare Moon hadn't happened... ponies were essentially free from conflict. Imagine, ponies without conflict... Nothing for them to struggle against, to learn and grow from...
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So, Discord took on the role of the devil, giving them something to force the ponies to grow and mature. (I believe that this was a time before most monsters as well; Discord clearly has had some influence on the everfree. I mean, timber wolves made of wood? Poison joke? Apples with dozens of arbitrary rules to preserve them longer than a couple of days?)
By the time real threats arose, ponies were stronger, ready to face them. I... don't think Discord planned on getting stoned for so long, though.
Doesn't it seem the slightest bit fishy that discord "betrayed" the main six, for someone who was bound to betray him just in time for him to learn a friendship lesson and give Twi a key?
When he spent the beginning of the episode practically pointing Twilight to the journal, WITH highlighted passages pointing out the five items collected so far?
It's almost like he KNEW what the keys so far were, and was going out of his way to get them the last one, neh?
I never said it was perfect.
My whole thing is that it was done FAR better than Glimmer's so-called "Reformation/Redemption"
Arguably, Starlight's not reformed.
In my headcanon, she just used the story about her friend and his cutie mark as a rationalization
She's not exactly sorry for what she did. She still tries to solve her problems by manipulating others, particularly if it can be done by magic. However, by pretending to be "reformed," and "sorry for what she did," she can learn more about friendship, and how it can be used to USE PEOPLE, with full sanction of the princesses, along with learning how NOT to cross Twilight and get taken down again.
In short, she's a monster, full on sociopath. But she's also adorably snarky. I hope they do take this path with her. MLP lacks true sociopaths.