Alias: Arvin Sloane was a permanent resident of the Heel-Face Revolving Door for four and a half seasons...until he murdered his own daughter, Nadia because she got in the way of his obsession with unlocking Milo Rambaldi's secrets. The show itself treats this act as the crossing of the moral Rubicon, and afterward Sloane is never treated as anything other than the Big Bad again.
All in the Family: David Dukes guest stars as a young man who, while posing as a police detective, wins Edith's trust as he comes into the house and describes a rapist that is terrorizing the neighborhood ... and it turns out that person is none other than himself! The live audience can be heard groaning as he crosses the MEH.
In a later interview, Dukes said it was hard to get the tone right for a comedy show. He wanted people to understand how things can turn very quickly and it wasn't going to be a funny experience.
Arrested Development: The matriarch Lucille has gone back and forth on how much she really cares for her family. It was Michael's job to try and keep the family together and the company afloat, and a lot of it was done for his parents' approval. In the final episode, when Michael learned his son George Michael was missing he was prepared to leave an elitist party to go looking for him, and Lucille scolds him for considering it, saying it would be rude to the guests. Michael, realizing what kind of person she really is, said "I've made a huge mistake." and left the party anyway.
Battlestar Galactica (Classic): Baltar goes out of his way to show what a monster he is by ordering the Cylon Centurions under his command during the genocide of his own people (whom he had sold out to the machines in exchance for being allowed to live and rule over his own colony like a king). He tells them to massacre all humans after a Centurian tells him that, up to that point, they had been making deals with prisoners for their lives.
Tom Zarek is morally ambiguous character who is difficult to label as an outright villain, right up until he orders marines to execute the Quorum of Twelve when they refuse to support his and Gaeta's mutiny.
Cavil was already pretty evil when he planned the genocide of humanity, but in The Plan he crosses the line when he stabs the young orphan boy who's been hanging around him because he realizes he's developing sympathy for the boy, which is undermining his hatred of humans. In other words, he crosses the horizon on purpose.
Cain. She always had a twisted sense of morality, but she completely went past the moral event horizon when she finds out her lovers a cylon, all but telling her crew to gang rape her ex-lover.
Surely Cain's MEH was when she ordered the execution of the families of civilians who wouldn't leave their now stripped-and-defensely ships to join the Pegasus.
Beverly Hills 90210: In the sequel series of this show Adrianna Tate-Duncan crosses the MEH when she tampers with her bipolar former friend Silver's medication as part of an escalating war over their mutual love interest Navid. Her previous actions, such as sending a naked picture of Silver to the entire school and poisoning her with tainted tap water, were bad enough, but once she's willing to endanger Silver's health and sanity over a boy, it's hard to see Adrianna as anything but a monster.
Big Time Rush: Crossed in the episode Big Time Sneakers, when Jett Stetson and Jo's publicist plan on faking a relationship between Jett and Jo, regardless of her relationship with Kendall. Jett and Jo's publicist flat out tell Jo and Kendall that since Jett and Jo are an In-UniverseFan Prefered Couple, so them dating gives the In-Universe show New Town High good publicity, and Jo refusing to date Jett will be the end of her carrer. So yeah, they tried to strong arm two teenagers into breaking off their relationship over tv publicity.
Hans Schroeder from the pilot crosses it when he beats his heavily pregnant wife, Margaret, so badly that she miscarries. Even resident mobster, Nucky Thomspon, is so repulsed that he orders Schroeder's death to serve as a fall guy in a botched heist.
Manny Horvitz started off as an affable gangster who reacted surprisingly reasonably to Jimmy avoiding paying back a debt. Even his decision to kill Jimmy after Jimmy sent an assassin after him is understandable. However, Horvitz completely crosses the line when he brutally murders Jimmy's completely innocent wife, Angela even as she says she can make Jimmy pay more.
Gyp Rosetti is by far the most psychotic gangster in the series, as such he has a few potential moments that can act as candidates for when he crossed the Moral Event Horizon.
Savagely beating a good Samaritan to death after the man had helped repair his car, just because Gyp saw the man as being condescending. This is also his to Establishing Character Moment.
Bombing Babette's Supper Club in a botched attempt to assassinate Nucky Thompson and Arnold Rothstein. While most gangsters can be ruthless and cold-blooded in the pursuit of profit and power, Gyp's bombing was on a completely different level, showing a disregard for civilian casualties never before seen on the show. Gyp calling Nucky the next day to mock him about Billie Kent being killed in the explosion was just icing on the cake.
Burying his right-hand man, Tonino's, cousin up to his head in sand and planning on leaving him to drown when the tide came in. Like his first victim, he did this only because he interpreted the man as being condescending. When Tonino begs for mercy since the man is family, Gyp obliges only to bash the man's skull in with a shovel then telling Tonino that he "owes him one."
While Warren Knox seems like a rabid Knight Templar FBI Agent, considering the people he's after, his actions can be seen as understandable. Then he detains, abuses and manipulates the relatively innocent Eddie Kessler so badly that the man commits suicide afterwards, and you know he's scum.
Dunn Purnsley seemed like a classic case of Defeat Means Friendship when he reappeared as Chalky's Number Two. Then he betrays his boss, aligns himself with Dr. Valentin Narcisse, distributes heroin throughout the Northside community, and cheerfully knifes an innocent priest to death when he attempts to tell Chalky what's going on.
Breaking Bad: Fans generally agree that Walter White has crossed the line from Anti-Hero to Villain Protagonist by now, although just when it happened is a matter of debate. Popular candidates include:
"Over", where he forces Walt Jr. to drink tequila until he vomits. While this is far less horrible than many of the others, it has the distinction of being the first time he did something terrible for no conceivable rational reason, but just because he could.
"Phoenix", where he stood by and watched Jane slowly choke to death during her sleep
"Full Measure", where he emotionally blackmails Jesse into murdering a defenseless man in cold blood, to save his own skin
"Face-Off", when he poisoned a child (albeit non-lethally) as part of an elaborate scheme to get Jesse on his side against Gus.
"Say My Name", where he shoots Mike in a fit of rage after blackmailing proves unsuccessful.
"Confessions", just when you thought he couldn't get any lower, he makes a "confession tape" implicating his brother-in-law, Hank, as the mastermind behind everything.
"Ozymandias" begins with Walter giving up Jesse to the Nazis, then telling Jesse how he let Jane die, even though he could have saved her. Towards the end of the episode, he kidnaps his infant daughter.
Selling meth to recovering addicts in Season 3 with Skinny Pete and Badger.
Gus' Moral Event Horizon came when he threatened to kill Walt's entire family, including an infant girl in "Crawl Space".
He may have given the order to kill Tomas, in which case he crossed it in "Half Measures".
Walt crosses the line for his whole family in "Ozymandias", when they learn that Hank is dead and assume Walt killed him. Upon phoning Walt Jr. in "Granite State", it is made clear Walt Jr can't be convinced otherwise and never wants to see his father again. In "Felina", he is seen talking to Skyler, who doesn't seem to mind him talking to her to say a last goodbye but still thinks he killed Hank.
Todd crosses this when he murders a child at the end of "Dead Freight", and again in "Granite State" when he kills Andrea and forces Jesse to watch, as punishment for Jesse's attempt to flee the Neo-Nazi compound.
This example is particularly interesting in that Spike himself saw it as a Moral Event Horizon; it made him realize that, without a soul, he was an amoral bastard who would hurt even someone he claimed to love. He didn't want to be a monster like that anymore; regaining his soul was his attempt to backpedal over that horizon, becoming a person with a conscience who would never hurt Buffy that way again.
Whenever a villain tortured a member of the Scoobie Gang, they tended to be seen as having gone just plain too far. Such as when Angelus tortured Giles to get information, and when in Angel, Faith tortured Wesley for no reason at all. Angelus was an actual villain, whereas Faith was just an Anti-Hero...
For the first half Season 6, The Trio are presented as little more than incapable comic relief, posing no real threat to The Scoobies or society. The murder of Katrina however, cements Warren as a full-blown misogynist with no care for anyone (including his lackeys) and no chance for redemption.
Word of God says that Angelus's murder of Jenny Calender in S2s Passion was important for the purpose of displaying how evil Angelus had become. Before that act, Angelus had murdered at least 4 people since being turned, but had not yet committed an offense so grievious to the audience (and the Scoobies) that it became a serious question as to whether or not it was even possible to redeem Angel, and if it was would anyone (besides Buffy) want to do it?
Chuck: Vivian Volkoff betraying her father Alexei Volkoff and leaving him to die, taking a bioweapon with her, which she fully intends to use. The significance of this stems from the fact that up to this point, the audience was led to believe that she was taking over the family business in order to get her father back, but this event and her dialogue during the act show that she's in it for the power and her hatred of Chuck for what is now essentially no reason.
Although "evil" is not part of the equation in either case, when Chuck witnesses Sarah cold-bloodedly execute a FULCRUM agent and later Sarah sees what she believes to be Chuck committing his first cold-blooded kill, albeit one he was ordered to commit, not knowing Casey actually pulled the trigger, both feel the other crossed the horizon and it negatively impacts their relationship for a while.
Dallas: Oh, Cliff, on account of practically murdering your own unborn grandchildren. By causing an explosion on a rig which your own daughter was and was pregnant with twins and still going on with it. For many you would stop and say "no way". But he pauses and thinks about it. Yes, you've crossed it hand down.
Degrassi: This show is generally more concerned with redemption, though Rick even lampshades his own: "It's too late. I already shot someone."
Dexter: Not really applicable to the title character, but for Miguel, definitely the murder of Ellen. Total violation of The Code.
And how about Lila killing Doakes?
Doctor Who: Word of God confirms that in the serial The Time Meddler, the Vikings did, in fact, rape Edith.
In Dragonfire, Kane has the tourists, passers-through, and residents herded into a spacecraft and blows it to Kingdom Come.
In The Curse of Fenric, Millington locks two men up in a cellar, leaving them to their Haemovorey death.
One could arguer "Logopolis" for the Master. Yes before he had manipulated, threatened, and killed lots and lots of people, but compared to the number of people The Doctor had manipulated, threatened and killed, they were basically even, and before Delgado died he was even supposed to have a Death Equals Redemption plot. And then, when he gets a proper new body again, he destroys one-third of the universe, including the home planet of one the Doctor's companion. And the new body he got is the corpse of said companion's father. After that, there was really no going back for him.
The Name of the Doctor and its followup, The Day of the Doctor reveal that an incarnation of the Doctor was forced to cross the horizon in order to end the Time War. In doing so, he refused to accept the name Doctor, and his later incarnations effectively disowned him. Day of the Doctor, however, reveals that events played out differently than the Doctor remembered, and he never actually crossed the Moral Event Horizon at all.
Drag Race: In season 3 of Ru Paul's show, Mimi Imfurst left a bad taste in everyone's mouth because of her melodramatics, whininess, inability to sew, and lack of professionalism but hoisting up India Ferrah despite her screams to be put down during the lip synch definitely cemented her scrappy status.
Firefly: Viewers can usually tell when a villain is about to die horribly when they threaten Kaylee or River. Dobson threatened both of them in the pilot, and it did not end well for him.
In "War Stories," Niska crosses the line when he kidnaps Mal and Wash and starts torturing them to death. This brings the wrath of the rest of the crew directly down on his head.
Made even worse when we realize Mal actually dies from it and he brings him back to life so he can continue torturing him.
Game of Thrones: As a show filled with morally ambiguous characters vying for power, many individuals cross it.
Most people in Westeros view Jaime as having crossed it when he broke his oath as a Kingsgaurd knight and killed Aerys Targaryen during Robert's rebellion, an action which earned him the derisive nickname of Kingslayer. He later admits to Brienne the real reason he broke his oath to protect the King: Aerys was planning on burning King's Landing to the ground rather than let Robert and the rebels depose him.
Joffrey was always a bastard with a Kick the Dog moment or two in season one, but he only really crosses the event horizon when he orders Ned Stark's execution, after assuring Ned's daughter and his future fiance that he would "show mercy.", which leads the North to rise up against the Iron Throne and secede from the realm. When confronted about his promise to be merciful he coldly replies that giving Ned such a quick and clean death was a mercy. His actions after are a long series of kicking dogs whenever there isn't someone else to rein him in.
Stannis crosses it in the eyes of Renly's supporters like Loras Tyrell and Brienne when he has Melisandre conjure the shadow creature to murder him. He later admits that he feels he crossed the line as well.
In the eyes of Robb Stark and the Northern lords, Theon crosses it when he executes Bran and Rickon Stark after capturing Winterfell for the Greyjoys. Of course, in reality he had two orphan boys murdered and tarred to pass off as Bran and Rickon, in order to keep the people of Winterfell in line. Which might be even worse.
The Freys and Boltons cross the Moral Event Horizon when they stage the Red Wedding, murdering Robb, Catelyn, Talisa, and the majority of the Northern lords. This is particularly egregious of the Freys, according to the customs of Westeros, since the Starks and their bannermen were their guests who had received food under their roof.
"The Boy" aka Ramsay Snow crosses it with his long, drawn-out torture of Theon Greyjoy, starting with his little "game" where he flays one of Theon's fingers until Theon begs him to cut it off. As Ramsay himself admits, he's not torturing Theon for any particular reason save one, because he enjoys it.
Littlefinger crosses it when he facilitates Ros's death by selling her to Joffrey, leading to the girl being used as target practice for Joffrey's new crossbow.
Gregor Clegane, already established as a reprehensible piece of work, crosses it when he has two hundred Northmen prisoners killed just to give the middle-finger to Robb Stark and his army.
Heroes: In volume 3, you're being subtly led to believe that you've been too quick to judge Sylar. When Peter Petrelli gains Sylar's ability for a while, he goes from saintly nurse to Ax-Crazy and nearly kills his own mother on sight out of the hunger for power before restraining himself, which makes you wonder, especially since to get that power Peter had to go to a future where Sylar is an upright family man and has undergone a total Heel-Face Turn. You also get to see that Sylar was so remorseful over murdering his first victim that he tried to commit suicide. He goes to Elle, whose father he killed, to plead for forgiveness and goes through what seems to be a Love Redeems subplot with her. Then he randomly decides he's had enough of being a nice person and kills her all because of random soap opera shenanigans that barely have anything to do with her. At this point you concede the point the writers were trying to make - that is, that this man is not right in the head. You also want him to die.
Sylar blamed her for helping to make him a killer (although it was mostly Noah's doing.)
Emil Danko: If it wasn't for ordering his men to open fire on illegally-abducted captives after their plane crashed, or for arranging Tracy's escape and sacrificing one of his own men to justify the government's practice of warehousing "specials," it was for removing Daphne Millbrook from the medical facility, resulting in her death from complications (sepsis) from a gunshot wound she sustained on Danko's orders (see above).
Samuel Sullivan: Depending on whether or not you subscribe to A Million is a Statistic, he either crossed it when he destroyed a town in a sinkhole or when, to regain the carnival's leadership after this depraved act, he pretended to surrender himself to Noah Bennet, then had Eli shoot up the carnival, killing Lydia (the only other carnie who knew he killed Joseph) and framed Noah for the whole thing. Samuel on other occasions had sinkholed a police station and a mansion but the people there were sufficiently "demonized" that those were Kick the Dog moments in contrast with the massacre of a town full of people.
Holocaust: In this miniseries, Erik Dorf crosses the Moral Event Horizon when he orders Karl Weiss tortured. He knows that Karl Weiss is the son of the man he once trusted as a friend, yet he still does it. Karl ultimately dies as a result of the torture. Before Dorf was sort of sympathetic, but after this, it became extremely hard to sympathise with him.
Homicide: Life on the Street: Detective Kellerman crosses it, not with the Vigilante Execution of Luther Mahoney, which was arguably justified, but a scene a few episodes later in which he makes light of a murdered drug dealer, even posing for a picture with the young man's corpse. From that point on, he stops being portrayed as a Cowboy CopAnti-Hero and becomes just an unlikable, self-centered Jerkass whose eventual departure from the police force is mourned by no one.
Ballard: We speak for the dead, remember?
Kellerman: Screw the dead. What have their moldering asses ever done for me?
Horatio Hornblower: In this mini-series, Jack Simpson crosses the line by trying to murder the titular character and another midshipman during a raid. He was despicable before that, but shooting one of your shipmates in the head just after setting adrift the jollyboat that another one is lying unconscious in? That's just evil, even if they both survived. Especially as the latter case resulted in said shipmate being presumed dead by his friends, captured by the enemy, and imprisoned for what appears to be a couple of years.
For Senkhara, it was Cursing Nina's grandmother. She had also been revealed to had murdered King Tut while she was still living...
House of Cards: In case his psychotic Machiavellian behaviour hadn't tipped you off, the TV version ends with the murder of Roger O'Neill and Mattie Storin by the Villain Protagonist Francis Urquhart MP, all in pursuit of the leadership of the Conservative Party and thus Prime Minister-ship. After this, the character is hard to see as anything other than sheer, concentrated evil. But stylish evil.
House of Cards (US): Underwood is evidently an unpleasant man right from the pilot but the murder of Peter Russo is a game-changer. Arguably the later murder of Zoe Barnes is even more shocking and completely objectionable.
iCarly: Nora and her family gleefully holding Spencer hostage to keep the iCarly gang imprisoned, torturing him if they do anything they don't like. What really pushes this over is Nora's implying that she can and will kill him if they make a wrong move.
She could've also crossed it when she tried to kill Gibby in iPsycho.
Nevel Papperman crosses it in iFight Shebly Marx. After the titular Shelby Marx, who's a female boxer, gets over an argument of Carly accidently pushing her grandmother down, she promises to go easy on her. Nevel would have none of it due to a petty vendetta he has on Carly, so he frames Carly for intetionally pushing down Shelby's grandmother, in hopes that Carly will be injured.
This one is also Played for Laughs but proves a point in iPity Nevel, after Nevel is seemingly redeemed, he yells at a man in a wheel chair for bumping into him for bumping into him and refusing an apology. Now Nevel has done worse, but it proves that even if he tries to redeem himself, he can't fight his nature.
I, Claudius: This is a series populated by devious conniving bastards who get away with some pretty horrible acts, but one of the worst dog-raping examples is provided by Praetorian Guard captain Macro when his predecessor Sejanus falls out of favor with the Emperor. Macro kicks off a bloody purge of everyone even remotely connected with Sejanus. Rome's streets run red, but the icing on the cake is when he orders the death of Sejanus's (very) young daughter. An officer reminds him that it's unlawful to execute a virgin. His response? "Then make sure she's not a virgin when you kill her, now GET ON WITH IT!"
Messalina might have cheated on her husband and become the biggest whore in Rome. And yeah, wanting to sleep with her stepfather, and then accusing him of trying to rape her when he rejected her, sure was bad. But she didn't reach the Moral Event Horizon until she threatened her own mother!
Bo Crowder crosses it when, in revenge for his son, Boyd, destroying a shipment of drug supplies Bo was meant to receive, he massacres Boyd's loyal followers and strings up their bodies for the Boyd to find, solely to rub salt in the wound.
Coover Bennett is a Dumb MuscleMan Child for the Bennett family, but he manages to cross the line when he tries to murder the fourteen-year-old Loretta McCready just because he was jealous of the attention she was receiving from Mags Bennett. First, however, he plans on showing her the shaft where he tossed her father's body down before throwing her down herself.
Dickie Bennett seems like a stupid but pitiable Big Bad Wannabe. Then he murders the relatively innocent Helen Givens and tries to arrange for his Dirty Cop brother to frame his remaining accomplice for the deed then murder him afterwards to ensure his silence.
Fletcher "The Icepick" Nix seems like a perfect example of Evil Is Cool before quickly revealing his true colors as a sadistic Psycho for Hire. Robbing a man known for his skill in using a gun, Nix arranges a "duel" between the two of them, with the rules being that after counting down from ten, both contestants will try to grab a gun placed in the middle of a table. He even orders a pizza so he could abduct the deliveryman and force him to act as the referee for their duel. Once the countdown is done, however, Nix cheats by impaling his opponent's hand with an icepick, then shoots both him and the referee. He basically turns a harmless robbery into a double homicide solely For the Evulz.
Nicky Augustine, a high-ranking Detroit mobster, crosses it in his Establishing Character Moment. One instant he's casually joking around with a childhood friend, who had been using his position in the FBI to help Augustine through Augustine's life, and the next he shoots said friend dead for failing Theo Tonin. Granted the victim, Barkley, was a supreme asshole, but it's chilling the way Augustine offs him with barely a change from his constantly smiling expression.
Danny Crowe crosses it with his casual murder of Jean-Baptiste merely because the man stood up to him and told him to stop bullying Danny's own younger brother, Kendal.
In Kickin' It, The Black Dragons, the main antagonists of the series, have crossed the line multiple times
Sensei Ty has crossed it in the past, continuously blaming Rudy for causing him to fall even though it was the Grandmaster who tooted.
In Wrath of Swan, Frank crosses it when he has Brody bring Kim to Swan Cotillion, and then fires gravy on to Jack and Kim
Sensei Ty crosses the line again in Kickin It on Our Own, by being brutal to the Wasabi Warriors, taking away Jack and Kim's black belts (he obviously had no right to do that), and forcing Eddie and Milton to drink heavy amounts of water to fight for their right to use the rest room. None of this grants Sensei Ty any points whatsoever.
LOST: We all knew Martin Keamy was bad news from the get go, but he firmly crossed the line in "The Shape of Things to Come", where he coldheartedly murdered Ben's daughter, just for the sake of proving he wasn't fucking around. Needless to say, he is quite possibly the only Lost character to ever cause Ben Linus to lose his cool.
If you think Locke's father, Anthony Cooper, hasn't crossed this by Season 3, "The Man From Talahassee" will obliterate that theory. He murders the son of one of his con victims for threatening to expose him, and when Locke confronts him about it, Cooper throws him out of an eight-story window, shattering his spine.
The producers purposely used this trope with the Man in Black, saying that they meant his causing Sun, Jin and Sayid's deaths to mark him as irredeemably evil. Then the show offered his Freudian ExcuseStart of Darkness flashback.
Don Draper on Mad Men has a lot of moments that make him irredeemably horrible — but probably the biggest is when he is caught cheating on Megan with Sylvia by his daughter Sally. The crossing of the horizon doesn't fully occur though until he chooses to explain the situation away to Sally by saying he was "comforting Sylvia". Sally doesn't believe him. Big shocker.
In the Malcolm in the Middle episode "Malcolm Defends Reese", Malcolm's scheming principal, Lionel Herkabe, who kept dragging Malcolm into his schemes only for Malcolm to get wise to his act before they can succeed, crosses it when he rigs Malcolm's grades by humiliating his brother Reese simply because Malcolm's GPA was about to exceed his. It's probably not a coincidence that after Herkabe got humiliated at the end of the episode, he made no further appearances on the series.
Merlin: Morgana spent from a year away from Camelot in the company of her half-sister and returned as The Mole, having performed a Face-Heel Turn in the interim. Over the course of the third season, her plots to bring down her father and half-brother have intensified in brutality, but it's not until Queen of Hearts that she crosses the line and ends up framing her servant and former best-friend Guinevere for witchcraft. Why? Because she had a dream that Gwen would one day become Queen of Camelot. Up until that point, fans were capable of some degree of sympathy for Morgana's Well-Intentioned Extremist views, but after seeing her smiling to herself as a terrified Gwen is hauled away to be burnt at the stake, the general consensus became: "the bitch must die!"
Uther from that same series passed the horizon before the series even started. He committed the "Great Purge" in which he hunted down and killed anyone with magical blood, even drowning children of magical parents in fear that they inherited magical blood.
NCIS: Recurring antagonist Ari crossed the MEH when he murdered Caitlin Todd, not to further any mission objective, but purely to cause pain to Gibbs and the team.
We later learn that Ari's father, Mossad Director Eli David, had crossed the event horizon decades ago when he deliberately raised his son to be such a monster. It turns out not to be the worst thing Eli has ever done, either.
Nikita: Percy most definitely crossed the MEH with the The Reveal that Kasim, the terrorist who killed Michael's family, was a Division agent who did so on Percy's orders in order to both infiltrate Al-Qaeda and make Michael so desperate for revenge that Percy was easily able to recruit him.
Once Upon a Time: The show seems to defend most dark characters as redeemable but greatly flawed, and plays with the trope interestingly.
Regina crosses for some fans after killing her father, who she truly loved, in order to enact the Dark Curse. But she was manipulated by an all-powerful immortal being who watched her grow up and made sure she would do it by making her miserable and insane. Actually, it's always hard to pick just one for Regina, but also very hard to agree on one, since in addition to the fact she was Forced Into Evil using her unstable mental health, was manipulated a lot and is sometimes Obliviously Evil to an unbelievable degree (when she orders her guards to hunt villagers, she realizes too late what that means, and sees in horror that they also targeted children), she geniuely tries to atone... and then fails due to the same problems.
Regina's mother, Cora, was weapons-grade evil. While you can argue about her double-cross of Rumplestitskin, removing her own heart because she loved power more, then marrying a doormat of a lesser prince and bearing a daughter (Regina) for the sole purpose of using her to gain more power is just the start of the list.
Some believe that Rumpelstiltskin crosses the line by vowing to enact a curse that will take away many people's happy ending, by asking a woman to cast it by harming the last person who trusts and take care of her most likely because of romantic rivalry, hurting an innocent fairy godmother, playing on people's sadness, and making sure they buy into terrible unbelievably shocking deals to attain his goal. It all depends on how justified you find this goal: finding his son that he abandoned and regretted doing so ever since.
Peter Pan, however, is pretty much on the other side of the Moral Event Horizon when introduced, as he has already deliberately and remorselessly abandoned his own son for eternal youth, kidnapped children and imprisoning in Neverland, manipulating or outright forcing people to do his bidding, and intends on tricking his own great-grandson into killing himself so that he can take his heart and use it to become immortal.
As of the second half off Season 3 Zelena seems to have crossed this when we find out she tricked Baelfire into sacrificing his own life to bring back his Father. And that she did it so she could get hold of his dagger and control him.
One Tree Hill: Dan Scott murdering his brother Keith in cold blood, and then pinning it on Jimmy Edwards, who had taken a gun into Tree Hill High & ultimately killed himself. Much like the Spike example in Buffy the Vampire Slayer, this is notable as Dan feels the guilt of what he did & tries to atone for his actions.
Power Rangers Samurai: In the episode "Trading Places" the Monster of the Week Switchbeast goes around city and turns innocent people into various objects. The point of this plan is that unsuspecting humans will get rid of those objects, unaware that there are human souls in them. This would have resulted in a lot of people being killed (including Rangers, Bulk and Spike) if Mike and Emily didn't defeat Switchbeast. That was the entire point of Switchbeast's plan: to have as many innocent people turned into objects and would have been disposed and destroyed by unsuspecting humans. Usually Nighloks want to make misery, but this time they wanted to kill innocent people.
In the eyes of Dayu, Big Bad Master Xandred crosses this when he sets her harmonium (which was a gift by Deker, when they were married and it was LAST remnant of her love for Deker, who was cursed and lost his memory of her) on fire and attempted to have her memory erased. She even says "I knew you were evil, but this!"
He really crosses it by killing and absorbing her after she revived him.
Serrator crossed it 200 years ago by making a Deal with the Devil with Dayu, turning Deker into a half Nighlok and taking his memories, not telling her that until she's already made the deal. If he didn't cross it then, he most certainly did with his endgame. He wanted to split open the earth, spilling the Netherworld onto Earth, likely causing untold death and destruction all for the sake of becoming powerful enough to rule both worlds.
Lightspeed Rescue's Queen Bansheera does truly horrible things. She crosses the line when Olympius, her son, ends up stuck in the local equivalent of Hell; she opts to leave him there for being useless enough to get stuck there in the first place, and seems to take perverse pleasure in doing it, too.
Venjix from Power Rangers RPM crosses this territory when he DESTROYS 99% OF HUMANITY. Also when he gains control of all the hybrids he experimented and hid in Corinth there are children among them and they don't even look a year over ten. Everything Venjix does qualifies for this trope.
While they are not as bad as Venjix, Alphabet Soup crosses the line at some point, either it is when kept super intelligent people, including children, imprisoned for their entire lives under the belief they've got some horrible illness that'll kill them if they step foot outside, or when they allowed Venjix to escape by not letting Dr. K install the firewall to stop the virus from spreading or finally when they tried to assassinate Dr. K to cover up Venjix's origins.
Whatever Monroe did that turned Miles against him must have been really bad.
Major Tom Neville crosses this in episode 11, because he showed no qualms about systematically killing off all the rebels. Then when his son Jason raised reasonable human objections over this and refused to call in the air strike, Neville tried to bully him, then he beat the stuffing out of him, and disowned him.
In episode 17, Rachel reveals that she has no qualms about letting a boy die just so she can get her revenge against Monroe. She would even abandon Aaron to do that.
The episode "Happy Birthday, Mr. President" reveals via flashbacks that Hollis and Cyrus crossed this before the events of the show, because Hollis killed 7 people and framed someone for it without remorse to cover up "voting irregularities", and Cyrus just shrugged it off when he was told about what happened.
The episode "Blown Away" ends up having Becky, Huck's girlfriend, cross this. First, she has the President shot and Huck is set up as the fall guy. Now, some people, like Huck, might not see this as crossing a line. But the minute he tries to take her down, she retaliates by murdering the entire family that Huck keeps an eye on. They were as innocent as they came, which makes her irredeemably evil.
The Shadow Line: Gatehouse crosses this in the third episode. When he was introduced, it was as an ambiguous and slightly sinister character, but definitely the lesser of two evils when compared to the obviously psychopathic Jay Wratten. But then he murders Andy Dixon, an innocent man set up as the Fall Guy for a murder he committed, and his mother and pregnant girlfriend just to ensure no witnesses remain, and it's clear that he's in fact a very ruthless and dangerous man. He becomes the main antagonist for the remainder of the series.
Shameless: Frank Gallagher is a morally reprehensible man who would rather pursue constant means of scamming people out of their money in order to feed his alcohol addiction, than to take care of his six children who are left to fend for themselves. He's done a lot of horrible things, including sleeping with an underage girl and attempting to sell one of his children for money. In the context of the show, however, even these actions are presented as things that he might still come back from. However, when he cons a dying woman into agreeing to marry him in order to get her pension when she passes away, then conceals from her the fact that a heart transplant has become available for her, allowing her to die from her disease in order to get her money that is an offense that would make him irredeemable in the eyes of many viewers.
The Shield: Had always played fast and loose with the moral event horizon concept with Vic Mackey, what with him shooting a fellow cop in the pilot and all. But his decision in the second to last episode to betray his only remaining friend, Ronnie Gardocki, by turning state's evidence against him and his cold proclamation that he would have no problem whatsoever LYING to Ronnie about his impending arrest, ultimately pushed Vic towards the point of no return for many fans.
Shane Vendrell from the same series had his own Moral Event Horizon moment when he murdered his best friend Detective Curtis "Lem" Lemansky, to ensure he did not turn against the Strike Team after being busted by IAD. Though the writers later tried to backpedal on this point of no return, by way of having Shane defend his actions by having Shane successfully own Vic's ass by way of lampshading Vic's own murder of a fellow police officer, for many fans it cemented Shane as the show's main villain for its final two seasons.
Shining Inheritance: In this Korean Drama, Eun Sung's stepmother is first seen as a somewhat strict, money-worried woman, but not so bad. Then, in several episodes she has thrown both of her step-children out onto the streets, without giving them a penny of their newly-deceased father's life insurance, and using lack of money as an excuse while she had enough money to buy both her and her daughter a large apartment and left her mentally disabled step-son, who she had found when lost, at an orphanage halfway outside town because she didn't want trouble AND arranged things to her advantage, lying to Eun Sung and manipulating her and saying they should act like strangers for the sake of her reputation because if the boy her daughter liked found out she kicked Eun Sung out of the house, he might get a bad impression of her. Oh, did I mention this all happened in 6 or so episodes? And the woman is still proud to live?
Also, until episode 15, Seung Mi, though not liked by a number of fans, didn't have a mob after her. Then, she lied about Eun Sung's circumstances, making him think Eun Sung was a liar and con artist, even though Eun Sung had done so much for her — and her reason was simply to not let Hwan think badly of her — this started several hundred conspiracies for her quick and painful death.
Shokojo Sera: The two main tormentors of Seira in this Japanese drama, Director Mimura Chieko and the Alpha Bitch Maria. Seeing how they are counterparts for Miss Minchin and Lavina in A Little Princess, it was no surprise. Having a huge inferior complex for Seira's deceased mother who she was classmates with, Mimura Chieko absolutely despises Seira for being too much like her mother and often slaps her when Seira stands up to her when no one else does. She is brutally cold when telling Seira about her beloved father's death and does nothing to lessen the pain, letting Seira know right off the start that she could kick her out of the school (leaving the poor girl without any shelter or food) and does so eventually.
Just as bad (or possibly worse), Maria is a Rich Bitch who likes the fact that she has control over all her classmates. But when Seira comes into the picture, she takes every effort to make her life miserable for easily stealing away her popularity. When Seira loses her fortune, she delights in making Seira grovel on the ground, donating huge amounts of money to the school and thus gaining enough power to make Seira her own personal slave maid. She would also purposely spill soup on Seira and throw tomatoes at her when Seira was already at the lowest point of her life. And in a heartwrenching Hope Spot for Seira, Maria makes her believe that there was a slimmest chance that Seira could be Juliet, something Seira has dreamed of for the longest time. Maria gets to be Juliet and forces Seira to work extra hours in the kitchen, not allowing her a chance to even be in the play.
Smallville: Lex Luthor has several moments of varying severity. It depends on when and how you deem someone "irredeemably evil".
In "Subterranean", casually walking by a series of prison cells holding meteor freaks in his secret lab, codenamed 33.1.
In "Freak", has his people abducting Chloe to said secret lab then experiment on and painfully humiliate her. He then swears to Lana upon his unborn child's soul that he has nothing to do with it, before watching a video of Chloe stripped half-naked and strapped to the experiment table. As she struggles, he delivers this line with a hint of Psychotic Smirk:
Lex: Regarding our most recent subject...keep a close eye on her.
This is especially notable for being directed by Michael Rosenbaum, Lex's actor, who has always wanted Lex to be evil.
The Reveal later in Season Six that he drugged Lana with synthetic hormones to fake the pregnancy and deceive her into marrying him, because he wanted to take her away from Clark forever. In "Promise", on the day of that very wedding, he even murders the doctor who helped with the deception, due to the man getting sick of it and threatening to tell Lana.
Forcing Clark and Lana into a heartbreakingSadistic Choice that leaves them separated by a bomb's worth of Kryptonite in the Season Eight episode "Requiem".
Davis crosses this when he kills Jimmy in Doomsday.
Major Zod. Either choking Faora, along with his unborn child, to death when she refuses to join him in conquering Earth or burning Tess with heat vision. In the case of the former, even he sees this as the point where he can't go back and condemns himself to the slippery slope.
Sonny With A Chance: In a 2-part episode, Penelope crosses this by framing Sonny for stealing, accusing her of plagarism, turning her friends against her, getting her booted off So Random, leaving Chad, Nico and Grady in a plane in a stormy sky without any parachutes, and for the icing on the cake, tries to kill Sonny with a bomb, all this because Sonny loved Chad.
Sons of Anarchy: Jax Teller's attempt to create a permanent break with girlfriend Tara by sleeping with porn starlet Ima served as a Moral Event Horizon for many. Regardless of his reasons or if he begs for forgiveness in the future, it's hard to believe Tara could ever forgive him. Too, while fans may be able to accept a Jax who kills people, runs guns and sometimes drugs and generally walks on the wrong side of the law, blatant infidelity is an irredeemable act.
Spooks: In series 9, we've been treated to watching Lucas' life falling to pieces as he tries to keep his shady past a secret in the face of ever-increasing blackmail from Vaughn, but he crossed the Moral Event Horizon fully in episode 6 when he hangs up a 999 call when Daniella lets slip she knows about Albany, sentencing her to bleed to death as he lies about the ambulances being on their way.
And if that's not enough, the next episode reveals he was knowingly responsible for both the bombing of a British embassy, killing 17 people, and the murder of a friend to steal their passport and identity. Not much chance of coming back from that....
Star Trek: Deep Space Nine: The last episode has the Female Changeling deal with Cardassian saboteurs by nuking Lakarian City; the resulting death toll is two million. When the Cardassian fleet learns of this, they perform a Heel-Face Turn, and begin firing on the Dominion and Breen ships. How does the Female Changeling react to this?
Female Changeling - "I want the Cardassians exterminated."
Weyoun - "Which ones?"
Female Changeling - "All of them. The entire population."
Weyoun - "That may... take some time."
Female Changeling - "Then I suggest you begin at once."
Gul Dukat was a mad, genocidal, sexually-ravenous dictator to begin with... and then he allied with the Dominion just to get back into a position of power, effectively setting off the chain of events that got his own planet razed to the ground.
Admiral Leyton spends most of "Homefront/Paradise Lost" as a Well-Intentioned Extremist who truly believes he's doing a good thing with his conspiracy to take over Earth and put it under martial law. He even looks sorry when he frames his old protege, Ben Sisko, and has him put in a holding cell. Until three-quarters through the story, the station captures another conspirator and is bringing him to Earth on the Defiant to testify. Leyton tells his right-hand-woman that the ship was taken over by Changelings and needs to be destroyed.
Kodos the Executioner from "The Conscience of the King" had murdered half the population of a colony world, picking the survivor half with Social Darwinism having convinced himself it was the only way to alleviate a famine
Star Trek: Voyager: The aliens in "Scientific Method" come across as an entire civilization who crossed the Moral Event Horizon long ago and have just kept on going. They routinely do medical experiments on sentient creatures, mutilating, torturing them, and even killing them if they feel it will benefit their medical research to do so. They feel completely justified in their actions and not only do they feel no remorse or regret over their actions, they feel that what they do is noble and beneficial. Genetically deforming, maiming and killing the crew of Voyager is the Nightmare Fuel evidence of their crimes and that is only the tip of the iceberg. What is really terrifying is that their flimsy justifications allow them to murder entire societies with impunity and go on torturing and killing as many sentient creatures as they feel is necessary for their "research."
Captain Ransom in easily crossed this line when he started murdering aliens as a fuel source for his ship. I Did What I Had to Do is nowhere near a sufficient excuse, but he at least seems to realize this at some level and eventually under goes a Heel-Face Turn and Heroic Sacrifice.
The Trickster/Gabriel, appeared to cross this for some people in Mystery Spot when he tortured Sam to insanity by killing his brother repeatedly but, in an unusual example, reversed everything and later became an ally, even if he was the sort of ally they wanted to punch. And who died for them.
Castiel ends up crossing the horizon in Season 6 when he breaks the wall in Sam's head, causing him to remember his time in hell, and then absorbs all the souls in Purgatory and declares himself GOD.
Dean crosses the Moral Event Horizon when he kills the kitsune girl that was friends with Sam-and had saved Sam's life. She had killed humans, true. Yet they were criminals who were actively hurting other people with their actions. And it was all to save her son who had caught an illness. Dean doesn't care. All he sees is a monster whose killed humans and he guns her down right in front of her son.
Affably Evil in prior seasons, Crowley plunges into the Horizon at transwarp speed, culminating in planning to hunt down and kill everyone the Winchesters ever saved just because he could, and to deprive the boys of the comfort of knowing that those people are alive because of them.
In Engine Sentai Go-onger, Big Bad Yogoshimacritein crosses the line when he shoots at the Rangers through his sympathetic subordinates Kitaneidas and Kegalesia, resulting in their deaths. He ended up shooting himself in the foot by doing this, however, as they hang on just long enough to destroy the source of his powers.
Samurai Sentai Shinkenger. Being a half-Gedoushuu who spends much of his time in his natural human form, Fuwa Juzo is subjected to many What Measure Is a Non-Human? topics, thinking he might make a Heel-Face Turn (he even likes Genta's sushi). His sword Uramasa is made from his family who wanted him to stop being a Blood Knight that lives in slicing people with it. Then, when Akumaro tried to use his human emotions to use Uramasa to create a Hell on Earth, Juzo instead slices him off and reveals that he prefers to be a full-blooded Gedoushuu and doesn't care one bit for his family's pleads, all he wants is to use Uramasa to give him the pleasure of killing people.
Juzo's successor in this role is Basco ta Jolokia from Kaizoku Sentai Gokaiger. Again the twist is that the expected twist of him really being not so bad never happens. Though presented as a traitor and villain who'll stab anyone in the back to get what he wants, he usually has a human face, and he's a lot of fun to watch, and he even has the same gimmick as Daiki Kaito, aka Kamen Rider Diend, who is a textbookJerk with a Heart of Gold. Okay, so he's not the Sixth Ranger after all like we all assumed, but surely he'll show his former friends some mercy when about to strike the final blow, or realize that there are more meaningful things than serving his own greed, right? Right? Well, surprise: it turns out he's a traitor and villain who'll stab anyone in the back to get what he wants. Including putting a bomb on his cute monkey-beast sidekick Sally and pushing the button, killing him/her instantly, to try to get rid of the Gokaigers, which is the moment that really makes you hate the guy. With this, he's revealed as actually being the evilest bad guy in the series (Compare this to the actual Big Bads, where Oiles Gil just wants to prove he's competent, Damaras is a Noble Demon, Barizorg's brainwashed, etc. Would any of them pull something like that?)
Before that, there was Choujuu Sentai Liveman and its Big Bad Great Professor Bias. To explain his Moral Event Horizon, it is important to understand how he runs his group, Volt. As of Episode 22, his students are three geniuses from Earth and two aliens, all of whose experiments he urges on with the utmost confidence. The first of the five students to go is the alien Guildos. Right before his death, Guildos realizes that he is not an alien, but a robot. Bias then confirms that not just Guildos, but the other alien, Butchy, were robots created to make the Earth students work harder. Butchy breaks down at this point because he had memories of a happy life that he just learned were completely fake and that his whole life was a lie and, as Butchy starts undergoing a Heel Face Turn, Bias makes him self destruct. If you don't consider this Bias's Moral Event Horizon, then there's his motive for everything in the series: He recruited four students from Earth- one of them had already made a Heel Face Turn by Episode 22- so he can raise their "scores" to 1000 so he could extract their brains and extend his immortality. After all the care he seemed to show to his students before this was revealed, this is shocking.
Teen Wolf: Quite a few villains do pretty horrible things.
Kate Argent seduces an adolecent Derek Hale and burns down his home, killing most of his family.
Because of this the first season's Big Bad, Peter Hale, crosses it before the series started by killing his niece for her Alpha powers.
Season two's Big Bad, Gerard Argent crosses it like a daily bridge - he declares a werewolf genocide on Beacon Hills, threatens to kill Scott's mother unless he betrays Derek's trust, forces his son to help kill his wife for being bitten. He uses Allison's grief to turn her into an almost perfect copy of Kate. Even when he kills Matt, the Kanima master, we feel more sorry for Matt. In the season finale, he beats up Stiles in an effort to hurt Scott and uses the Kanima to hold his granddaughter hostage - all this was an elaborate plan to become a werewolf himself to cure his own cancer.
Despite their Magnificent Bastard status, the aliens in The Monsters are Due on Maple Street cross the MEH by basically committing genocide by corrupting the targeted people.
In the original V, Daniel Bernstein steadily becomes worse as he joins the visitors, using his newfound power for petty acts of cruelty and to threaten other people to get what he wants. He eventually kills an old woman who was spying for the resistance in the Visitor Headquarters, despite her pleas to him to let her go and how she's known him since he was a kid. The moment is portrayed as the point of no return for him, and the resistance sets him up to be killed when they raid his house later on.
Veronica Mars: In the S2 finale, it is revealed Cassidy blew up the bus at the beginning of the season. This might have been forgiven given his Freudian Excuse, but then he blows up the plane where Papa Mars was supposed to be and just in case that wasn't evil enough, he is revealed to be Veronica's rapist from S1.
Walker, Texas Ranger: Most of the villains tend to Kick the Dog half a dozen times before the episodes are over, but some villains really show their cruelty and cross the line. Examples:
Lazarus, vicious and merciless killer for hire from 4-part Story Arc is already shown as cruel and remorseless assassin, killing undercover cops in a lot of violent ways and showing no remorse for it, but he really crosses this line when he kills an innocent young boy off-screen and after that doesn't feel anything.
Johnny Blade from episode "The Lost Boys" just kept crossing the line: first he organizes the heist and kills a cop. Then he gives the gun to one of his accomplices and the same accomplice hides in his friend's house. After learning this Johnny threatens and innocent young teenager Jesse(the same friend who Johnny's accomplice hides the gun in his house and Carlos' nephew) to remain silent about his crimes and give him his gun back or else he will kill his mother. But he doesn't just stop there. Later Jesse is arrested and thought to have killed the cop and later Johnny kidnaps his mother and forces Jesse to take all the guilt guilt for his crimes and falsely confess or else he will kill his mother. But after Jesse does so, Johnny orders his lawyer and his henchmen in prison to kill Jesse, even after he took all the guilt and attempted to make his mother commit suicide. This was so evil, that judging from the look on his henchmen's faces, they seemed disturbed by it. Luckily he is defeated and his accomplices are arrested.
Recurring villain Victor La Rue is pure evil, in "The Trial Of La Rue" he takes the courtroom hostage, kills the judge, and taunts Alex.
The Walking Dead: Ed Peletier was certain nobody's favorite character, constantly abusing Carol and Sophia just to show his dominance. Nobody was sad when he was the first of the group to be killed off when the camp was invaded by Walkers and he was off sulking in his tent because Shane had beaten the shit of out him earlier in the day. However his Moral Event Horizon moment doesn't get revealed until the season two premier when Carol states that he was looking at his own daughter, suggesting he was ready to sexually abuse her as well.
Shane crosses a MEH when he shoots Otis in the leg, leaving him to be eaten alive by walkers. If not that, then he's definitely crossed it when he snaps Randall's neck in cold blood and attempts to frame the boy for attacking him, to justify it.
Stringer Bell leads the pack by ordering the murder of D'Angelo Barksdale in prison, then negotiating a drug deal with Prop Joe behind Avon's back at D'Angelo's funeral.
Walker is shown to be an all-around bastard, but crosses the Horizon when he breaks Donut's fingers rather than attempt to arrest him.
Valchek counts. He orders an investigation into Frank Sobotka over a personal feud which ultimately leads to Frank's death at the hands of The Greeks. Valchek doesn't seem particularly bothered when this happens, handwaving that "That's what happens when you lay down with gangsters."
Method Man's Cheese Wagstaff crosses it when he sells out his uncle, Proposition Joe, to Marlo Stanfield, leading to Joe's murder.
24: Several characters cross it, even when they're not intending to. In fact, an alternate name for the show could be "Moral Event Horizon: The Series". It's a very long list:
Season 1 villains: Kevin Carroll crosses it by smothering Janet York to death. Andre Drazen leaps over the horizon by killing a hostage after Jack has already complied with his demands, and finally, if his father Victor didn't cross the horizon while he was a warlord, he most certainly crossed it by killing his friend's daughter without hesitation or remorse when Jack takes her hostage.
Season 3: Ramon Salzaar shooting his brother Hector for not going along with a deal.
Season 5: Christopher Henderson likely already crossed the horizon before he even appeared on-screen by ordering the murder of David Palmer and Michelle Dessler. If not, then he seemed to do so via his implied murder of Evelyn Martin and her daugher.
Season 6: Abu Fayed detonating a nuke that kills many thousands of people, Graem Bauer either by his involvement in Day 5 or his confessed multiple attempts to murder Jack, Philip Bauer either by murdering Graem to cover his tracks or threatening to murder his grandson Josh, and Cheng Zi by torturing Audrey Heller for no other reason than to spite Jack and use her against him.
Season 8: Charles Logan by corrupting Allison Taylor to redeem his public image, and Yuri Suvarov, for orchestrating the day's events, particularly the murder of Omar Hassan and Renee Walker.
Up until the finale of the first season, Nina Myers seemed - at best - to be someone with muddled loyalties. Even when it was revealed in the previous episode that she was Victor Drazen's contact in CTU (and was thus responsible for tipping the assassin off to the safehouse where Teri and Kim were staying), she still wasn't overtly bad. That changes in the span of the finale, though, when it's revealed that she not only slit Jamey Farrell's wrists while the latter was handcuffed to a chair, but goes on to shoot and kill Teri Bauer (who just revealed to her that she was pregnant), despite not having any real reason to. Even though the writers tried to humanize her in the next two seasons, it didn't work.
Sherry Palmer appeared to be a Lady Macbeth-esque figure who's constantly scheming behind-the-scenes to make sure her husband doesn't lose the Presidency. That pretense gets dropped when she lets Alan Milliken (who was trying to blackmail her husband) suffer a heart attack while railing at him, and then preventing Milliken's wife from administering the medicine he needs.
For some, Mike Novick never recovered from the season 2 incident where he pushed Presidential assistant Lynne Kresge down a flight of stairs (which crippled her), then comforted her as she was being wheeled to the ambulance.
Well, actually Mike didn't do that himself. He just locked Lynne in a room and assigned a guard to watch her. The guard is the one the pushed her down the stairs while she was trying to escape. On the one hand, Mike did a pretty bad thing by locking her in a room so she wouldn't interfere with the plan to impeach Palmer. On the other hand, he likely had no idea something like that was going to happen to her. But yeah, the "comforting" thing still made it a lot worse...
The show's writers had attempted to humanize Habib Marwan in Season 4, via deleted scenes that show him interacting with a wife and child who he is very caring towards. These scenes were apparently dropped when they realized that he crossed a clear line when he killed tens of thousands via a nuclear power plant meltdown, then followed it up by blowing Air Force One out of the sky and killing the majority of people aboard (including the President's son).
According to some, even Jack has crossed this line, via his execution of an unarmed (and surrendering) Dana Walsh. He does get called out on it by other characters, and it seems to be the moment where he's finally crossed the deep end.
What really makes it all the more of one for Jack was the fact that in spite of his methods he still had a bit of a personal moral code that was always above the attitude of Revenge Before Reason. Killing Dana was essentially the point where he went "Screw it."
Additionally, many feel President Allison Taylor crossed it by willingly listening to Charles Logan, despite knowing all about his past crimes and the type of man he is, and outright threatening Dahlia Hassan with military invasion of her country.