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- For those who have played it, all that needs to be said is "Protocol 10." For those who haven't, Hugo Strange's plan to eliminate crime in Gotham City is a mass execution of every living person in Arkham City. For the record, Arkham City contains a number of "political prisoners" whose only crime was knowing too much about his plans. When Batman makes him look at the results, his only response is "It's glorious, isn't it?"
- Ra's al Ghul is definitely shown to be insane when he threatens to kill Talia, his own daughter, to make Batman kill him and take his place.
- Even though he's an Insufferable Genius, The Riddler's always been held up as one of Batman's lighter and less-sadistic villains. While he had put various security guards in elaborate death traps, at least there was always the chance of Batman being able to save them. That's why it's so genuinely shocking to see him put Cash and the other remaining guards in a death trap that will inevitably kill them, allowing him to watch them slowly walk and walk until they're too exhausted to continue and they get blown up by motion-sensitive bombs; all while he taunts them for being too ignorant to escape on their own. If you look around the control room, you'll find a "scoreboard" that shows a few of them had already met their end before Batman came to the rescue.
- Hush crossed this by murdering six innocent people, cutting off their faces while they were still alive, and making his face a perfect copy of Bruce Wayne's, all to ruin his life over a petty grudge.
- The Scarecrow already proves he isn't playing around when he threatens to douse the city in fear toxin on several occasions, but the moment he forces Gordon to lure Batman to him, then throws Barbara off the construction building when Gordon seemingly kills Batman is the moment he truly crosses the line.
- An in-universe example; the act that made Jason Todd decide that The Joker was utterly irredeemable and had to die was when he blew up a school filled with kindergarteners and crudely stitched their body parts together, with the children's mothers fighting over their "patchwork corpses". For one last even worse touch, Joker sings "Humpty Dumpty" to said mothers over the intercom to spitefully taunt them... and it's revealed that he was counting on Jason coming after him, setting a trap at Arkham Asylum to capture him, before horrifically torturing him over several months and ultimately leading to Jason becoming the Arkham Knight.
- Harley Quinn was never the nicest girl in town, but her actions in A Matter of Family were horrific, even if she only was an accomplice. Just to recap: she helped The Joker con the desperate rich father of a little girl with terminal cancer into building an amusement park for her just so the Joker and Harley could use it as their secret hideout once the father and daughter died; helped turn said little girl into a test subject for the TITAN formula with the promise that it would cure her (which also doubles as a Moral Event Horizon in retrospective for Dr. Young from the first game), but was what caused her painful death; drove the grieving father into suicide by rubbing salt into the would AND gave him a drug that would supposedly kill him quickly and painlessly but was actually Joker Venom that gave him a horribly painful death between laughing fits.
- Garfield "Firefly" Lynns crosses it when he holds a dozen people hostage on the Gotham Pioneer Bridge with the intention of luring both Batman and most of the GCPD there, so he can blow it up and kill everyone once they're all gathered. Even when Batman informs the masked arsonist that the bounty on his head has been called off, Firefly acknowledges it... but continues his attempted mass-murder regardless, purely out of sadistic joy.
- We already knew how much of a deranged sadist Professor Pyg was, but his actions in the Gotham Story Dollotron firmly establish just how monstrous he can be. As part of a tour for his Circus Of Strange, Valentin abducts a young boy and agonizingly turns him into a mindless Dollotron through a series of brutal surgeries, proudly showing off his handiwork to the boy's father. The traumatized man offers Pyg a significant sum of money to just have his son back, at which point Pyg shakes his head and utters the following ominous line, before siccing a large group of other Dollotrons — several of whom are noted to have previously been children and their parents — on the man with the intention of inflicting the same horrors upon him.Pyg: Not your son now. My Dol-lo-tron.
- If The Penguin didn’t cross it by blowing up a bridge and leaving his own men to die (and laughing about it), he definitely crossed it when he shot a cop in front of Batman just to taunt him. Listening to his audio tapes reveals that Penguin probably crossed it long before the game started by torturing and murdering his own men in horrific ways and stuffing them up in his museum.
Injustice franchise
- Injustice: Gods Among Us
- Regime Superman crosses this line when he burns a hole right through the skull of Regime Shazam for daring to turn around on his word of a new world order. Granted, Supes had done a lot more prior as the High Councilor of the Regime, but when he does that, on top of giving orders to destroy Gotham City and Metropolis (the very cities he had gone Knight Templar in order to protect from another tragedy like the one that took Metropolis and launched his Start of Darkness) to punish the people that are siding with the Insurgency, it provokes Regime Flash to pull a Heel–Face Turn. It is this action that establishes Regime Superman as an irredeemable tyrant that has truly crossed the line.
- Regime Superman killing Green Arrow is the moment he truly crosses the Moral Event Horizon, as it is the first time he kills another hero.
- The Joker crosses it big-time with the act that gave birth to the Injustice-verse — namely, tricking Superman into killing his wife Lois and their unborn child by taking them up to space without a suit because he was under the effects of a Kryptonite-induced fear toxin that made Superman think she was Doomsday. To make things worse, this triggers a Dead Man's Switch that Joker had placed in Lois, which would set off a nuke that Joker had planted in Metropolis, devastating the city and killing millions. And then he goes on to drive Superman to murder him, something that he was never able to do with his archenemy Batman, in what would prove to be the catalyst to this Superman's Start of Darkness, sending him down the road to tyranny and evil in the name of preventing another such tragedy from ever happening again.
- Regime Superman crosses this line when he burns a hole right through the skull of Regime Shazam for daring to turn around on his word of a new world order. Granted, Supes had done a lot more prior as the High Councilor of the Regime, but when he does that, on top of giving orders to destroy Gotham City and Metropolis (the very cities he had gone Knight Templar in order to protect from another tragedy like the one that took Metropolis and launched his Start of Darkness) to punish the people that are siding with the Insurgency, it provokes Regime Flash to pull a Heel–Face Turn. It is this action that establishes Regime Superman as an irredeemable tyrant that has truly crossed the line.
- Injustice 2:
- If Poison Ivy betraying her longtime friend Harley and joining the Society wasn't bad enough, the moment she uses her pheromones to take control of Harley and nearly poisons her with said pheromones is when she really crosses it.
- Gorilla Grodd does it by allying with Brainiac and selling out Earth, not caring even for Gorilla City possibly getting destroyed. Even the villain group he formed disbanded because of this reason.
Other Games
- LEGO Batman: Lex Luthor easily crosses it by trying to murder everybody in Wayne Tower, because Wayne snubbed him at the Man of the Year Awards.
- Batman: The Telltale Series:
- The Penguin crosses it when shooting the innocent debate moderator. Even Bruce admits this event cemented that his best friend is gone. If that wasn't enough he tries to kill Harvey by smashing his face with a stage light as opposed to the quick death he gave Mayor Hill who actually deserved it.
- Sadly for Batman/Bruce, Thomas Wayne crossed it by deliberately subjecting Esther Cobblepot to a Fate Worse than Death just to get her family's estate.
- Vicki Vale, of all people, crosses this by forming the Children of Arkham, getting heaven knows how many people killed, and later drugs Bruce to get him out of the way.
- Riddler already crosses quite a few lines in his introduction — mutilating Mori with intent to execute him, slitting a casino security guard's throat for standing in his way, ordering his men to shoot their hostages to serve as a distraction, cutting off his prison warden's face in old security footage — but the act that solidifies him as a true villain of the Batfamily is indirectly smuggling a signal-broadcasting puzzle box into the Wayne Enterprises building, which leads to the manslaughter of Lucius Fox when the pulse leads a homing missile straight into the side of the building. For no reason other than a sick compulsion to test the Batman's mettle, Riddler manages to hurt over a dozen innocent people, and kill one of Bruce's oldest friends and allies, all in one stroke.
