- When Dolores Umbridge takes over, she spends most of Harry Potter and the Order of the Phoenix finding new and more creative ways to Kick the Dog (usually Harry). For her moral event horizon, take your pick:
- When she forces Harry to write lines using an enchanted quill that repeatedly carves the words into his own hand - until it won't stop bleeding;
- Her horribly cruel manner of dismissing Sybil Trelawney;
- When she tries to fire Hagrid;
- Sending several spells at an unarmed McGonagall when she tries to interfere with her firing Hagrid; or
- Her trying to literally torture (i.e. use the Cruciatus Curse on) Harry for information.
- It turned out that she sent dementors after Harry.
- Amazingly, she manages to get even worse in Deathly Hallows.
- Also crossed when Bellatrix, Barty Crouch Jr., and two other Death Eaters subject Alice and Frank Longbottom (Neville's parents) to a Fate Worse than Death prolonged torture by Cruciatus curse until they became insane, unrecognizable vegetables. It's even worse that they did it after Voldemort was gone.
- Wormtail is an instructive example of where true event horizons lie. While betraying Harry's parents to Voldemort, supposedly his best friends, causing an explosion that killed a dozen innocent people and pinning his mass murder on Sirius, ensuring Sirius' twelve-year psychological torture in prison was certainly evil, he could have still been redeemed. After the initial betrayal, he at least had a meager chance to start his life anew, probably in another country. But after he found Voldemort in Albania, helped restore him to full health, killed Cedric, and resurrected Voldemort, he truly cemented his place in darkness and ensured that the casualties would be much worse than those he caused himself.
- Vincent Crabbe, along with Goyle and Malfoy were mostly ineffectual rivals to our trio, but towards the end Crabbe actually almost kills the three, even when Draco is dead set against it. In the movies, due to Crabbe's actor's troubles with the law, this line crossing was given to Goyle.
- Good luck guessing when Uncle Vernon crosses the MEH, though it was probably in Order of the Phoenix after Harry saves his son Dudley's life from a Dementor. Instead of thanking Harry for such an act, he shamelessly attempts to kick Harry out of the house upon his realization that the enemies who've been on the hunt for Harry might actually (gasp!) pose a danger to anyone else in his household, even though he knows his nephew will most likely be murdered in cold blood if he's left without a safe place to stay. Only a Howler from Dumbledore, and Aunt Petunia's subsequent intervention, convinced him to back down.
- Considering all the shit the Dursleys put Harry through all his life, for such petty reasons, the line has been crossed long ago, to the extent that Harry who suffers from Chronic Hero Syndrome was seriously wondering whether or not he would save them from Voldemort, in case they were kidnapped. From Harry, this means a lot
- Lucius Malfoy was a willing accomplice in many of the Death Eaters' atrocities, but the moment where he slipped Tom's diary to Ginny, knowing that it would kill Muggle-born children, he proved to be capable of great vileness of his own accord.
- In the film adaptation, Harry tricks Lucius into freeing his house elf Dobby. Lucius almost uses the Killing Curse on Harry, only for Dobby to stop him from doing so.
- Also, he participated in the sadistic torture of Muggles during the Quidditch World Cup.
- Gilderoy Lockhart toward the end of Chamber of Secrets, when he tries to give the twelve-year old Harry and Ron severe, irreversible memory damage and leave Ginny to die in order to keep secret the fact that he's a fraud.
- Bartemius Crouch Sr. crossed it (only in the novel) when he disowns his son during his trial. Even though he was guilty, before his son did anything horrible, his son tried everything to please him countless times. Not to mention, he didn't even care about his son one bit.
- He definitely crossed it when he permanently damaged Bertha Jorkins' brain via Memory Charm. This is where you can tell that he's no better than his son.
- Snape himself is already a jerk who bullies the Gryffindor students, mostly notably Neville. However, the final straw is when he kills Dumbledore. Subverted in Deathly Hallows when it's revealed that it was an act of mercy since Dumbledore was dying very painfully, which Dumbledore himself arranges. Though one could argue that the abuse of innocent children already crosses the line.
- Voldemort may have crossed it when he tortured the children in the cave as a boy, or when he killed another boy's rabbit, or when he made his first Horcrux after killing Myrtle Warren. From the reader's perspective, he crosses it in the first scene of the series when he murders Harry's parents and then tries to kill Harry himself, a defenseless infant.
- Antonin Dolohov's murder of the Prewett brothers is this for him despite being a Minor Major Character.
- For Fenrir Greyback, it was turning Remus Lupin in a werewolf when he was four years old, condemning him to a life of painful transformations and discrimination from society.
- Fantastic Beasts: Corvus Lestrange crossed it years ago by using the Imperius Curse on a married witch to have her for himself, having a child with her in what is essentially rape.
- In-Universe, creating a Horcrux is considered to be this, as it requires to commit murder and...something horrific in order to split one's soul and keep it in another object. It's considered to be so bad that even books that go into great detail about The Dark Arts will not describe it, and it's implied that the discovering that Voldemort has created Horcruxes is what caused Regulus Black to undergo a HeelFace Turn.
Moral Event Horizon / Harry Potter