The Shredder kills Master Splinter.
Leonardo: You killed him! He was your brother, and you killed him! (Between exchanged blows) He was your brother, Saki! Your best friend! How could you do that?!
Shredder: Splinter was cursed to become a loathsome rat! It was his fate... HIS DESTINY!
- In the season 8 premiere, Shredder actually destroys the Channel 6 building which takes it out of commission for the rest of the series.
- Long before that, there was Shredder intentionally exposing April to the lethal fumes of the Doku plant back in "Enter: The Fly," which very nearly killed her. And this was back in the second season, which had already been made Lighter and Softer than season 1!
- In "Shredder Triumphant", Bebop and Rocksteady try and shove the Channel 6 crew out of a window.
- In "Return of the Technodrome", Krang tries to destroy New York with an earthquake after its populace refuses to vacate the city so he can use it as a base of operations.
- Shredder's personal Moral Event Horizon comes in the episode "Mission of Gravity", when he orders his lieutenants to steal the Triceraton engine holding the city of Beijing aloft miles above the earth, despite the fact that doing so would naturally doom the millions trapped within it. It was possible to remove the engine without killing anyone, but he didn't care.
- When Leonardo saves Karai from falling to her death, she stabs him from behind with his own katana when he would have let her go in the spaceship anyway. This is one of the things that leads him to tell her she is just as bad as the Shredder. It can be argued that every decent thing she did after that was just self-preservation.
- Technically in that instance, Shredder punched him into the blade, which Karai held at that moment. If you look at it for a split second she's visibly shocked by what just happened. So in that case it's TECHNICALLY an accident.
- Karai's alternate future self from "Same As It Never Was" aided Shredder and served him as his right-hand, even when Shredder took over the entire planet, turning it into a massive hellhole, enslaved humans and Utroms, planned to invade other worlds and reduced much of Earth's population and killed everyone who opposed him in brutal ways. Yet she still aided him and never even expressed any remorse for any of this, while stating that she just follows her duties, but at this point this argument to defend herself is meaningless. She is just as accountable for this as the Shredder. Karai from present at least questioned his acts and even went against him in "Mission Of Gravity" and pulled a HeelFace Turn during the Turtles Forever, when she found out that Shredder was attempting to erase the entire universe.
- In Turtles Forever, 2003!Shredder callously attempts to murder the Turtles by using them to pinpoint Turtle Prime. Granted, he's tried to kill them hundreds of times before... but not while Splinter was watching them die, yes, the other Shredders have attempted this, but there was no real sign of a Deus ex Machina, Chekhov's Gun, or really anything this time that could save them except somebody from the outside, this is what gets Karai to finally switch sides for good.
- Shredder crossed this when he killed Hamato Yoshi's (this was before he was mutated and started calling himself Splinter) wife, burned his house down, and took Yoshi's daughter Miwa to raise as his own, naming her Karai (meaning "weapon"), blaming Yoshi for murdering her mother, and making Karai hate her biological father. Why did he do all this? Its essentially because he was jealous of Yoshi and he just kept going.
- Near the end of the first season, he allies himself with the Kraang, making a deal with them that would result in everyone on the Earth dead or mutated, just so he could get Splinter out in the open. Then it's revealed that Shredder didn't kill Splinter's daughter, Miwa, he kidnapped her and raised her as his own - Karai. Meanwhile, he fed Karai a lie that Splinter killed her mother, giving her a vendetta against Splinter as well. He's been using her as a pawn for revenge her ENTIRE LIFE, and she doesn't have a clue. And he doesn't hesitate to sic her on Splinter when she comes to help Shredder.
- What makes this even worse is that he's outright shown he has no compassion at all for Karai. She's simply a means to an end. Even the fact she genuinely loves him means nothing so long as he can use her to hurt Splinter. The big crossing in this regard comes when he responds to her asking if Splinter is her real father by saying yes and having her locked up. No show of sorrow or upset. No feelings of remorse. He just basically tells her 'yes, I used you as a living tool of revenge' and discards her.
- Him blaming Splinter for Karai's mutation probably counts as a better example, but for different reasons. Her mutation was more or less entirely his own fault, with Splinter having nothing to do with it. In a way, Shredder actually caring about Karai makes it worse because instead of going My God, What Have I Done?, he uses it as more fuel for his hunger for revenge instead of accepting responsibility for his actions.
- He most certainly crossed the horizon for good in "Annihilation Earth" when he throws away a chance to save the world AND Karai in favor of killing Splinter through a literal, figurative, and possibly symbolic backstab, which leads directly to the Triceratons sucking Earth into a black hole.
- Slash aka Spike at first came off as a Well-Intentioned Extremist because Raph was mad at his brothers. However, what he did to the other Turtles against Raph's wishes puts him beyond the pale.
- If siccing a mind controlled Splinter to kill the Turtles wasn't enough, Rat King crossed the line by using his so-called rat brothers as pawns in capturing humans.
- In "Trans-Dimensional Turtles", the return of 1987!Krang. He sets bombs in 2012!Earth, 1987!Earth, and Mirage!Earth to get rid of the Turtles forever, even going as far as to dupe his "cousin", Kraang Sub-Prime, that he was helping them conquer their worlds. Subprime did NOT take Krang being revealed to be intending to destroy the exact kind of dimensions Kraang Prime and company have spent eons trying to mutate very well.
- Lord Dregg shows how utterly irredeemable he is in the fittingly-named "The Evil of Dregg". Having his armada ready to attack the Salamndrain homeworld, he forced Mona Lisa and Sal Commander to betray the Turtles by luring them into a trap. He then reveals to the two he had no intention of sparing their planet, having plotted on eating them the entire time.
- Za-Naron crossed it when she temporarily kills Donatello while possessing April's body, in her quest to wipe out all life on Earth.
- Jei first crossed it offscreen by murdering an innocent family. Then, it's revealed that he wants to use Kintaro's body as a vessel for his spirit.
- Years ago Eric Sacks killed April's father, who had set fire to the Project Renaissance lab to prevent the Shredder from using it towards his own means.
- Dumber-than-a-sack-of-hammers they may be, Bebop and Rocksteady hit this when they nearly killed Donatello, Bebop holding down the turtle while Rocksteady smashed his back with a sledgehammer.
- Bishop doesn't cross the Moral Event Horizon so much as he kicks it in the ass and spits on it after he kills his own father, a defenseless elderly man, with zero hesitation.
- He gets a second one when he brainwashed Slash and implanted a nuclear bomb in him, forcing the poor thing to self-terminate.