Jafar from Aladdin crosses this when he leaves Aladdin to die in the Cave of Wonders, once the latter gives him the lamp.
In Atlantis: The Lost Empire, Commander Rourke, who planned to steal the crystal from Atlantis all along without telling Milo, punched the King of Atlantis, mortally wounding him given how old he is. It was enough for Dr. Sweet to turn against Rourke and try unsuccessfully to treat the King's wounds when everyone else was abandoning Milo and the people of Atlantis taking the crystallized Kida with them, even before Milo's sarcastic, fatalistic appraisal of the people he thought were his friends actually convinced Audrey, Vinny, Moliere, Cookie and Ms. Packard to join him and abandon Rourke, who had just crossed the line even further by punching Milo down for his "soapbox," and stepping on the picture of Milo and his grandfather, breaking the glass of the frame.
Gaston from Beauty and the Beast comes close to this when he tries to blackmail Belle into marrying him by threatening to have her father tossed in the loony bin. He comes close to it still when, out of envy of the Beast, he breaks down and stirs the town into an angry mob to make them kill him. However, Gaston finally crosses this by literally stabbing Beast in the back after Beast spared his life (and wearing a very nastysmile while doing it). He falls to his death soon afterward.
Lady Tremaine from Cinderella is established as a nasty piece of work throughout the film, and her deliberately breaking the glass slipper at the end is particularly cruel enough to be considered a possible candidate for her MEH since she did it purely out of petty hatred toward Cinderella (and she also shows no regard for the Grand Duke's life), but she truly seals the deal in the third installment, A Twist in Time, when she poofs Cinderella in a twisted pumpkin carriage with Lucifer as its human driver, and attempts to get Lucifer to kill Cinderella (especially heinous because previously, she had been satisfied with simply making her life a living hell). She also has Anastasia posing as Cinderella to fool the prince. If anyone doesn't know, this is rape by association. As The Nostalgia Critic put it:
Nostalgia Critic: She had no magical powers, no minions, all she had was complete control over one person's life. And man, did she squeeze every last drop out of it.
It's later revealed in the Tie-In NovelA Frozen Heart that Prince Hans' father, the King of the Southern Isles, crossed it long ago by abusing his 13 sons for his own ego so they'll be transformed into his sycophantic henchmen, aside from tormenting his subjects for not providing enough money or insulting him behind his back. And because Anna and Elsa wanted Hans out of their lives as soon as possible, they have no idea just what kind of person they sent him back to, meaning the king continues to make Hans' life a living hell.
Most Disney villains are goofy, somewhat quirky people, and even the darker and nastier types, such as Queen Grimhilde, Lady Tremaine, and Scar, had a funny and/or quirky moment or two. But not Judge Claude Frollo from their version of The Hunchback of Notre Dame. If he didn't cross this after almost dropping a baby down a well, he did after burning down multiple homes with families still inside them. The fact that Frollo uses religion to justify his actions only makes it worse. In the opening sequence, the music seems to indicate a crossing of the M.E.H. when he decides to kill baby Quasimodo. About five seconds after he says, "A baby?... A monster!" The music jumps keys and tempo as he moves towards the well, and is supposed to make you realize "Oh S*** he's going to kill a baby in the first five minutes of a Disney movie!", so there will be NO redemption for this guy.
Scar evidently crosses this by murdering his brother Mufasa. It even seems, from Scar's facial expression at the edge of the cliff, that he realized there would be no going back once he did this, yet he did it anyway. Further evil actions (such as tricking Simba into blaming himself, or sending the hyenas after him) were done to get away with murdering Mufasa, which makes that the most likely candidate for Moral Event Horizon. When Simba finally tries to use the same words against Scar that he used against Simba (to "run away and never return"), instead of complying, proceeds to engage in an epic battle with Simba. But just before Simba and Scar battle, the latter foolishly blames the hyenas for everything, effectively sealing his fate when Simba leaves him to them.
Ursula starts well over the line and really crosses when she interferes with her own deal with Ariel and hypnotizes Eric into falling in love with her in disguise. It gets worse when she attempts to blast Ariel with the trident, after turning into a giant version of herself.
Dr. Facilier of The Princess and the Frog has an entire plan that puts him over the line — in exchange for wealth and status, he's willing to sacrifice the souls of everyone in New Orleans to his "friends on the other side".
The Rescuers Down Under: McLeach, who kidnaps a young boy named Cody to try to get the location of some eagle out of him, crosses this towards the end when he ties Cody to a crane, lowers him into a crocodile-infested river, raises him back out again, and would dunk the boy in again until the power on his halftrack goes out. When this happens, he takes out a gun to shoot the rope, suggesting that he was originally intent on murdering Cody anyway and just wanted to torture the kid first.
Though for the most part he's as Laughably Evil as the average Disney villain, Prince John from Robin Hood crosses it when he plots to use Friar Tuck as bait to capture Robin Hood, knowing Robin Hood will save any friend of his who's in danger. Even Sir Hiss is shocked to hear that his boss plans to hang Friar Tuck. He further goes over when he tries to murder Maid Marian in the alternate, extended ending, which makes it all the more understandable when King Richard has his brother locked up and notes that were it not for their mother, he'd have him executed then and there.
Maleficent in Sleeping Beauty after she condemns Princess Aurora (as a baby no less) to her doom by pricking her finger on a spindle needle on her sixteenth birthday — and all because she wasn't invited to the party. Partially this and likely partially just for fun, with even the not being invited thing as a mere direction towards whom was she going to destroy. The movie leaves room for interpretation as far as her motives go.
Snow White and the Seven Dwarfs. After Snow White has grown up enough for the mirror to declare her more beautiful than the queen, the queen orders her hunter to kill her and bring back her heart as a trophy (she even already had a box custom-made for the occasion). When that doesn't work, it turns out she's studied murder spells as a backup plan. When she learns that the Sleeping Death spell she's put on a certain apple will only leave Snow asleep and can be cured, she isn't fazed but rather delighted — because she's sure that the dwarfs will bury Snow alive! Snow must have had her hands full as the new Queen — she probably had to sort out some pretty sadistic lawbooks.
Clayton from Tarzan crosses this when he shoots Kerchak, Tarzan's adoptive father, then, much like Gaston, tries to kill Tarzan after Tarzan spares him, resulting in a gruesome Karmic Death afterward.
Mother Gothel in Tangled officially crosses the line either
When she stabs Flynn Rider and very nearly kills him, so that Rapunzel's secret would die with him.
Or arguably even before already when she sets him up to be caught by the authorities knowing that he will be sentenced to death, which nearly happens, but he escapes that, only to be stabbed by Gothel as mentioned above.
Zootopia: Bellwether crosses it when she shoots Nick with the Nighthowler serum in an attempt to make him kill Judy. It's driven home when she chooses to stand there and watch Judy be hunted and killed, even giving a Psychotic Smirk when Nick clamps down on her neck.
Giles Winslow Jr. crosses it when it's revealed that he spent the weekend golfing instead of helping Christopher Robin work out the cost cutting at the company. It's here that it's revealed that the whole 'swimmer or sinker' spiel he gave to Christopher was nothing more than a manipulation tactic Winslow spun just to get someone else to do his work for him, all the while knowing full well that his own job security was almost assured because his father Winslow Sr. is the company founder. Quite mild compared to what you'd normally expect for this trope, but in the context of the franchise, that cemented Winslow's status as a villain. Fitting for someone whose name sounds similar to "woozle".
Christopher Robin himself believes he crossed it at some point during World War II, and even confesses as much to Pooh. Being forced to kill countless of your fellow humans can do that to you. More pointedly, he had lost a lot of innocence he will never truly be able to regain in full, and the movie is about him regaining what he'd lost.
The Jungle Book (2016) has Shere Khan confront Akela and the wolf pack about why Mowgli was not handed over to him. Akela says that Mowgli is going back to the man village, so there will be peace for the boy and the wolves. In response, Shere Khan...just goes ahead and kills him, all for Akela apparently denying him his prize. Whatever sympathy Shere Khan had for what humans did to him, it's cost him a lot of that.
From TheLizzie McGuireMoviePaoloValisari crosses it by manipulating Lizzie into posing as his look alike ex-girlfriend Isabella, and lip sync as Isabella at the International Music Video Awards, to ruin Isabella's career.
Captain Barbossa of the first movie and his men have done such a deed so many times when one would suffice as a MEH and their last time is the only time that viewers witness it first-hand, specifically the brutal opening attack on Port Royal that leaves many civilians dead, injured or just terrified and this includes toddlers.
Cutler Beckett, the new villain of the next two films, was an outright bastard and crossed the MEH around the time that he had a ten-year-old boy hanged for piracy or even just associating with pirates.
Pirates of the Caribbean: On Stranger Tides: Blackbeard crossed this when he had Syrena tied up half in the water for a slow, painful death to extract a tear from her. Phillip, who before stated that everyone had some good in them, admits he was wrong and there is no chance of redemption for Blackbeard.
TRON: Legacy: Clu is a dog-kicking program who takes pleasure in viewing and participating in games to derezz citizens in Tron City. It's not a question of if CLU crosses the line, but when. Here are some milestones:
Stealing Tron's light jet and leaving Tron to die in the Sea of Simulation after Tron refuses to shoot down the Flynns and crashed his jet into Clu's.
And even before all of that, the genocide of the ISOs. Since he viewed the ISOs as an imperfection (as well as threatening his ability to create a perfect system, which is exactly what he was made to do), he purposely reprograms an ISO into a virus to infect and derezz ISOs in Evolution. This was not just used as a distraction for Flynn while he takes over the Grid, it was also used as an excuse to turn the Basics against the ISO population.
Turning the Games lethal, when Flynn explicitly made the Games non-lethal, which just seemed to be a dick move to punish Programs that annoyed him (and/or piss off Tron).
And "rectification"...Taking and twisting sentient Programs into attack drones. And then, when you try factoring in how much work must have gone into Rinzler and the twisted delight he likely got out of it.
Who Framed Roger Rabbit: Judge Doom demonstrates the Dip by dipping the cute little shoe affectionately rubbing his boots. What makes this even scarier is that as the shoe goes into the Dip, smoke not unlike that which comes out of the smokestack of an incinerator starts coming out of the Dip as if Judge Doom was using the thing to burn that poor shoe alive.
And if that wasn't enough to make you hate him, his plan to annihilate Toon Town just to build a freeway, and then revealing himself to be the Toon that killed Eddie's brother probably will.
In A Bug's Life, Hopper probably crosses the line once he kills two of his own underlings to pass his message: it's not just about the food, it's about keeping the ants in line. And it all begun because a freak accident ruined the food originally set out for the grasshoppers, making Hopper think he needed to tell them "You Have Failed Me". And thousands, if not millions, of puppies continue to be kicked afterwards, as one of his goons reveals Hopper planned to squish the Queen after they finished the mass food theft just to teach the "lesson". When this is about to happen, and after suffering a No-Holds-Barred Beatdown at one grasshopper's hands, Flik confronts Hopper with a "The Reason You Suck" Speech ("Who's the weaker species? Ants don't serve grasshoppers! It'syouwho needsus!"), but this pushes Hopper even further past the line, especially when the ants decide to stand up to the grasshoppers. In the ensuing chaos, an enraged Hopper tries to kidnap Flik, but when his attempt is foiled, this pushes Hopper so far beyond the Horizon that there is no going back, especially when Hopper confronts Flik again.
Hopper:[rough with pure hatred] YOU THINK THIS IS OVER?!?!? ALL YOUR LITTLE SCHEME DID WAS BUY THEM TIME!! Flik: No, please! Please, Hopper! Hopper:[grabbing Flik's throat] I'll get more grasshoppers and be back next season! But you won't!
Coco: If Ernesto de la Cruz didn't cross it when he murdered his best friend Hector to steal his songs for the sake of becoming famous the second the latter had decided to quit the music career and return with his family, he definitely did when he doubtlessly threw both Miguel and Hector in a hole, in order to keep his reputation and fame in the world of the living intact, despite him fully well knowing that Hector would die for good if he gets forgotten, and that Miguel (who at that moment he believed to be his long-lost grand-grandson) will not be able to return home.
And if you still thought there was anything good or redeeming about him then Would Hurt a Child attempting to throw Miguel off a cliff to his death in order to prevent him from bringing Hector's photo, and the truth about his career to the living world, all of this while shamelessly claiming to be innocent will make you think otherwise.
In Incredibles 2, the villain crosses this when they create a decoy Screenslaver by hypnotizing a pizza delivery guy to be the scapegoat. When Helen challenges Evelyn on framing an innocent person, she displays a complete Lack of Empathy toward her victim and even justifies it by saying the pizza was cold and the delivery guy was surly.
Randall Boggs from Monsters, Inc. plans to kidnap small children so he can use the Scream Extractor on them, a device that sucks the scream and oxygen out of them, running the very real risk of suffocating them to death. And unlike Waternoose, who crosses the Horizon himself due to this same set of circumstances, he can't claim to be a Well-Intentioned Extremist, because his reason for doing it is petty jealousy against Sulley — jealousy which, as it turns out, goes back to the prequel, where it's revealed that all this time, Randall had been beaten out as Scare Leader by a college dropout, making his jealousy all the Harsher in Hindsight.
Toy Story 2 has the Prospector closing and locking the air vent after Woody says he wants to leave. Earlier on, the "how long will it last" speech seems reasonable enough, but resorting to locking someone in for not being convinced is clearly unjustified, and in turn makes much clearer whose perspective we're encouraged to listen to.
Ansem (actually Xehanort) definitely crossed this line in the original Kingdom Hearts when it was revealed that he experimented on living people just For Science!. And it was outright stated that the experiments caused their hearts to collapse!
Ansem's former self Master Xehanort may have did a lot of bad things throughout his life, but extracting the Darkness from Ventus's heart and creating Vantias (which almost led to the boy's death) really says a lot. But he may have already crossed it in his youth, when he travels into the future and destroys Sora's heart. Or earlier, when he almost suffocates the boy.
Saïx of Organization XIII crosses this when he coldly refuses to take Sora to Kairi, opting to use her kidnapping to make the Keybearer full of rage if it means collecting more Hearts to finish Kingdom Hearts for, what Saïx wouldn't know at the time, Xehanort's ultimate goal of using it to make clones of himself.
Terra-Xehanort crossed this when he uses chains against Aqua and Ventus during the Second Keyblade War, and tortures them casually, making it look like the two of them had been hanged. For Ansem's Guardian, this was the last straw, and breaks free of Terranort's hold to save them, which leads to Terra retaking his body from Xehanort's possession for good.
Multiple Disney Villains have crossed the line, but the moment they crossed it is often the same as in their source films. This is not the case with Jafar, Lady Tremaine, Scar, Clu, and Maleficent; all of them have committed acts so heinous that they're viewed as managing to become even bigger monsters in this franchise than they are in their films.
Jafar crosses this line when he orders Genie to show him the Keyhole for Agrabah. Had Sora not shown up, the end result of this would amount to genocide, as the world would've been destroyed by the Heartless, and everyone in it would have been killed, which is what Jafar wanted. He is far worse in Kingdom Hearts II, where he becomes an Omnicidal Maniac bent on destroying the entire universe.
Lady Tremaine and her two daughters cross the line together when they outright try to murder Cinderella with an Unversed which was summoned from their hatred towards her. The monster drops a bomb on them as part of their damnation, and Aqua implies they became Heartless.
Scar might be operating on a smaller scale than Maleficent, but he still manages to be her equal in viciousness. Aside from his canon atrocities, he crosses the line when he gleefully tries to eat Sora shortly after the later arrives in his world. What makes this particularly vile is Sora is in the form of a Lion Cub, so Scar essentially wants to cannibalize a child. It's also worth mentioning that he's the only character in the series so far other than Xehanort that's been able to keep his original form as a Heartless.
Clu crosses the line when he forces Sora to fightTron/Rinzler, then kills the latter after the battle.
However, all of this amounts to nothing compared to the actions committed by Maleficent throughout the entire franchise. She's single-handedly responsible for hundreds, if not thousands of worlds being destroyed, and the deaths of billions of innocent people; a record unmatched in Disney history until the Starkiller Base and Thanos. She's kidnapped girls who's hearts are pure light and forced them into deep sleep until all seven of them are brought together for universal domination (or worse, the end of reality itself). And she's even tricked Terra into stealing Princess Aurora's heart, and is in direct league with Xehanort.