These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Jerkass Woobie: Again, Discord. He's been through sheer hell, but he's not what you'd call "gracious" toward others.
Moral Event Horizon: An In-universe example. If Discord's conversation with Twilight in chapter 15 is anything to go by, he seems to view his accidental murder of King Aegeus and Queen Astraia as his personal crossing of it. To the extent that he basically flat out tells Twilight he's a bad person in spite of trying to bring his adoptive parents back.
Then there's the utterly horrible version of Discord in Fluttershy's nightmare. It's telling that when Discord comes into the dream and "saves" her, he's genuinely disturbed by the thing. It's also implied that this isn't the first time this has happened.
Discord gets a severe case of blood poisoning in an early chapter, and Twilight suggests turning him back to stone as a last resort. Celestia immediately shoots the idea down. Her reasoning is that Discord may just die in the stone, or even be stuck in pain forever, unable to die.
Then there's the basilisks. Sweet mother ofGod, the basilisks.
Twilight's Heroic BSOD after nearly killing Discord is both saddening and a bit frightening, given how deep she sinks into depression. Luckily, she eventually recovers.
Just try to read Chapter 6 without wincing. Poor, poor, Discord.
The entirety of the scene in Chapter 13 in which Discord comes to clean to Celestia about murdering their parents definitely qualifies. Especially the ending of the scene.
Discord confronting Luna in about the murders also qualifies.
The Woobie: Based on everything she’s been through, Princess Celestia could very well qualify for this trope. Let’s review shall we? Growing up, she was forced to imprison Discord, who was essentially the love of her life, in order to stop his madness, has to live with the knowledge that Discord was responsible for the death of her parents, has to keep this knowledge a secret from her little sister, who viewed Discord as a brother, and eventually had to banish Luna for 1000 years. And this is all before the story began. Throughout the story, it’s clear that Celestia really would like to reconcile with Discord, but his resentment over her imprisoning him, and his guilt over murdering her parents causes him to act coldly towards the princess. Until the 13th chapter that is.
Luna counts as well. Even Discord displays pity for her at one point and calls her a "misunderstood, frightened mess".