So I've been hearing a lot of people saying Discord is still evil and is not, and never will be, reformed.
Twilight's Kingdom - No one paid much attention to his betrayal in Twilight's kingdom because of the presence of Tirek, who was the first ever G1 villain, the epic DBZ fight with Tirek, and if anything the episode seemingly finalized Discord's redemption when he gave Twilight her key and the ponies finally accepted him, even Rainbow Dash and Applejack.
Make New Friends But Keep Discord - He threatened to send Tree Hugger to another dimension, but we all dismissed this as Discord still having a bit to learn about friendship, and merely character development for him just like any of the Mane 6 (especially Rainbow Dash).
But the big issue is in What About Discord. Though he didn't put the other Mane 5 under a spell as Twilight had suspected, he intentionally leaves Twilight out to teach her a friendship lesson. While his reasons seemed noble, and he said he didn't intend for her to feel as bad as she did about it, many here thought he was just doing it to be a jerk to her, saying the friendship lesson thing was weak and calling him "vile" while saying reformation isn't something that would fit with his character. This is what got to me.
I'm feeling really depessed about this because he was one of my favorite characters, and we already had to go through this with Crowley in Supernatural, he seemed to at least partially reform at the end of Season 8 and I wanted him to stay that way, but goes back to full-time evil at the end of Season 10 after Sam tries to kill him.