Steele from Balto seems to be your average animated movie Jerkass at first—making fun of Balto, flirting with Jenna even though she's not interested, etc etc. All par for the course. But when he refuses to let Balto take the crate of medicine, thus condeming the children who desperately need it to certain death, it becomes clear how horrible he is. And, once he was defeated, he deliberately tried to ensure that Balto and the whole team of dogs got lost and died, while he could return home and pretend to be a heroic survivor.
Darla Dimple and her servant Max from Cats Don't Dance crossed it when they sabotaged Danny's chance to impress L.B. Mammoth (or, more specifically, their method of doing so). They do so by flooding the stage with so much water that it practically drowns the entire studio (which, if this were realistic, would have killed lots of people), but instead, it sets up Danny and co as incompetent and causes them to abandon the production, less scary and not as bad as the other examples, but still a Jerkass move.
Lord Barkis Bittern in Corpse Bride crossed the line a long time ago after he murdered Emily in cold blood to steal her dowry.
Heavy Metal. During the Harry Canyon segment, the titular character helps an unnamed woman hide from a group of thugs who want the Loc-Nar from her. The following day, Ratnik, the leader, contacts Harry to meet with him at the Statue of Liberty and give the Loc-Nar to him in exchange for a huge sum of money, getting the woman out of danger. And how does the woman thank him? By pointing a gun at him and demanding the entire amount for herself. The 2 of them previously agreed to split it in half.
Although, the film implies it wasn't entirely her fault. The Loc-Nar is said to corrupt the will of people it interacts with, and bring out the evil potential within them.
In How to Train Your Dragon 2, if it wasn't clear enough by then that Drago Bludvist is 100% morally bankrupt, it most certainly was when he uses his Alpha to brainwash Toothless into killing Stoick.
In Kung Fu Panda, when Tai Lung confesses to Shifu that all he ever wanted was to make him proud, only to find out that he had Shifu's pride and love all along, he only spends one moment looking genuinely affected before he's right back to demanding the scroll, saying he doesn't care about apologies. Since he's basically thrown away any sort of redemption that might have been offered to him, that moment signals that he deserves everything that happens to him afterward.
At this point Tai Lung probably did not believe that Shifu was sincere (he'd just finished giving him a No-Holds-Barred Beatdown and was going to kill him—ergo, Shifu would say anything to save himself) and was past reasoning with him anyway, Tai's last speech during the battle was only meant to hurt. That's why they made Po offer him a Last-Second Chance later.
In the second Movie, Lord Shen, as far as he appears on the screen, always was a power-hungry murderer, but he really crosses the line when he massacres an entire panda village.
As if that isn't enough he coldly kills the Wolf Boss just because the Wolf Boss didn't want to murder his own men.
In the book, he crosses much, much sooner, although Soren/the reader doesn't know it at first. He shoves Soren out of the nest, hoping it will kill him (his own brother) then does the same to said little sister, and has a hand in his own parents' deaths.
Planes: Ripslinger sending Ned and Zed to sabotage Dusty during the China-Mexico stretch.
In The Powerpuff Girls, if you thought that Mojo Jojo was an incompetent and harmless villain in the TV series, be disappointed when you see The Movie. He arguably crossed it by enslaving Townsville with super monkeys, destroying half the town, and betraying the girls' trust in the process. Confronted with this development, he tries to Break Them by Talking:
Ironically, since the show is set after the movie, this trope is inverted, as Mojo peaked in his evil here and only had down to go, becoming MORE redeemable rather than less.
Pharaoh Seti in The Prince of Egypt led a campaign to curb the Hebrew population by rounding up and killing all of their infants. He's not too sorry about it, either, with murals depicting the event adorning the walls of his palace. When Moses, his adopted son, tries to call him out on it, Seti just blows off his concerns, saying "Oh, my son... they were only slaves..." He thought this would make his adopted son feel better. He was trying to comfort him with those words. Oops...
In South Park: Bigger, Longer & Uncut Sheila Broflovski starts out as a Well-Intentioned Extremist who works to ban Terrance and Philip's film Asses of Fire, and ultimately all Canadian products to solve the problem of children swearing. Even when she orchestrates the arrest of Terrance and Philip her actions are still understandable, if off the deep end. Finally, when Kyle helps rescue Terrance and Philip from execution and delivers a heartfelt speech to convince her to relent, she snaps and kills Terrance and Philip herself. This leads to an My God, What Have I Done? Moment when Satan tells her that her actions have allowed him to return to conquer earth.
Superman in Superman vs. the Elite jumps through the MEH when he decides to finally buy what Manchester Black is selling... by murdering each of the Elite, then lobotomizing Black by destroying his powers. Then, we find out he subverted it by revealing none of the Elite are dead, that an army of robots saved them, and that his way works.
Many Disney Animated Canon movies have the main villain cross the horizon at some point - mostly so that even small kids can tell who is the good guy and who is the bad guy, and determine that the bad guy DID deserve a violent death or, at the very least, a thoroughly spectacular form of Abject Humiliation Conga. Here are some examples, in chronological order:
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.
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.
That, or just for fun, with even the not being invited thing as a mere excuse. The movie leaves room for interpretation as far as her motives go.
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.
In The Rescuers, it's hard to tell when Medusa, who kidnaps a young girl named Penny to make her look in a flooded well for diamonds, crosses this, but it's evidently when she refuses to let Penny up even when Penny's life is in danger. Oh, and did we mention that Medusa makes several direct attempts on Penny's life with a shotgun when she tries to escape?
Likewise in its sequel, it's not that clear when McLeach, who kidnaps a young boy named Cody to try to get the location of some eagle out of him, crosses this, but it's likely 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.
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 closer 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 stabbing Beast after Beast spared his life (and wearing a very nastysmile while doing it. He falls to his death soon afterwards.
In The Lion King, Scar (picked up) evidently crossed 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.
Most Disney villains are goofy, somewhat quirky people. 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. And he kills Quasimodo's mother within the first few minutes of the movie! That means that he crossed the line shortly after the film began and just kept running. 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.
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.
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.
Dr. Facilier of Disney/The Princessandthe 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."
Mother Gothel in Disney/Tangled officially crosses the line when she stabs Flynn Rider and very nearly kills him, so that Rapunzel's secret would die with him.
And both stealing the food and plotting to squish the Queen were intended as a You Have Failed Me because a freak accident ruined the food originally set out for Hopper. Even worse, he originally decided against going through with it considering they had enough food to last them through the winter already, but then he remembered that Flik had stood up to him once before, and decides to take the opportunity to crush a potential rebellion.
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.
The Incredibles's Big Bad Syndrome is given a sympathetic history as Mr. Incredible's former number one fan, cruelly (in his eyes) rejected by his hero. However, he shows his true colors first by luring dozens of superheroes to their deaths under the guise of offering them employment and then later firing missiles at a plane that he was well aware had children on board.