Similarly, in Justice League, Flash's execution by President Luthor led to another alternate universe's Justice League becoming the Justice Lords.
In Batman Beyond, after getting a second chance at living a normal life, Mr. Freeze is betrayed by Derek Powers when his body begins to deteriorate back to requiring sub zero temperatures. It's around this point that he's crossed the line and eventually chooses to stay behind when the building is collapsing on top of him. Batman tries to save him but he seals himself in with a wall of ice. His last words in the episode? "Believe me, you're the only one who cares..."
The Beast crossed it years ago when his selfishness caused not only his own curse, but also led to his servants being punished with him. Belle helped him pull himself back across the threshold only for him to cross it again when he's forced to release her to save her father.
Moral Orel. Nature. Clay shoots Orel in the leg and leaves him bleeding in the woods for a day. It's when the show officially shifts from comedy to a massive character study.
Danny Phantom saw his friends and family die right in front of his eyes. What comes as a result? A completelypsychotic ghost who goes on a literal roaring rampage (not of revenge). Though admittedly, it didn't help that his ghost-half fused with Vlad Master's evil ghost-half, Vlad Plasmius. He's the only villain on the show to have been seen to commit murder.
During the Time Skip in Young Justice, Aqualad went through this when Tulla, the girl he loved, died during a mission with the team as Aquagirl and when he discovered Black Manta was his biological father and that Aquaman knew and hid it from him. Feeling betrayed by everyone in his life, he went to the only place he felt he could belong: at his father Black Manta's side. In "Depths", this is revealed to be a cover story for his Fake Defector status.
This incident was a big shock for the rest of the team too. Wally Artemis and Garth retired from heroics, causing them to bring in new heroes and change the line up, they brought in Mal to act as a constant mission control, and everyone else became very cautious and serious compared to the first season, their feeling of invincibility seemingly shattered.
In Adventure Time, Billy, an aged hero, reaches this after years of fighting evil proved futile, as the bad guys just kept coming back. As a result, he becomes an example of I Will Fight No More Forever, feeling violence didn't help solve any problems.
Finn almost crosses this in "Dad's Dungeon," after he's convinced that Joshua, his adoptive father, doesn't love him. He's about to commit suicide by eating a poisoned apple, but Jake saves him at the last minute.
Baby-Snaps crosses this in "Princess Cookie," after Princess Bubblegum giggles at his childhood dream of being a princess. He tries to commit suicide.
A more humorous example was used in "Crystals Have Power," when Jake makes a vow of non-violence after accidentally hurting Finn and his brother Jermaine, and becomes a complete wuss.
More serious was in "Incendium" when Finn spends the entire episode in the floor, clutching the picture of Princess Bubblegum. He got better when he met Flame Princess.
In The Legend of Korra, Korra hits this after Amon takes her bending away. However, this actually is what she needed to unlock her spiritual side which connects her with Aang allowing him to restore her bending and unlock the Avatar state.
Tarrlok also reaches this point after he has a My God, What Have I Done? moment and realizes that him and his long-lost brother, Amon (born Noatak), have become the same evil monsters that their abusive father wanted them to be.
Korra hits the event horizon again in season 2 after her uncle, having been transformed into a twisted version of the Avatar himself, rips the light spirit Raava from Korra's body and actually succeeds in bringing the Avatar cycle to an end. Tenzin manages to drag her back, but it's a narrow thing.
Tragically, Korra's third experience with this has even more serious, lasting consequences. Being poisoned by Zaheer and lingering in a near-death state for an extended period of time in the finale of Book 3 leaves her crippled, heavily traumatized and struggling with depression. It takes her over two years to recover enough to even walk, and even as Book 4 opens it's clear that she still has not fully recovered.
This ends up being the Freudian Excuse for Kuvira in Book 4. It's discovered that her actions were because she was abandoned by her parents and she attached her love for them to the Earth Kingdom as a form of coping. But after the Earth Kingdom was thrown into chaos, she became desperate to save it. It results in her becoming a tyrant.
It's thankfully avoided, but heavily hinted by Asami in the second to last scene of the series, that Korra dying would have caused Asami to completely fall apart.
In Teen Titans this happens to Terra when she reminds Beast Boy that he said that he'd be her friend no matter what happened. His response? "You don't have any friends." By her next appearance she's given up on good and is committed to killing the Titans and taking over the city for Slade.
Beavis in Beavis and Butt-Head Do America falls into this near the end. After arriving in Washington and not meeting Dallas Grimes (who had promised to have sex with them when the he and Butthead made it to Washington - a lie so she could get the toxin she put into his shorts), he delves into this, crying out that they'd grow old and gray, but they were never going to score. The bus driver makes Beavis Get a Hold of Yourself, Man! by delivering a No-Holds-Barred Beatdown.
Ahsoka, of all people, crosses it in the Star Wars: The Clone Wars in the finale of the fifth and final season, after the Jedi Council throw her out and almost convict her of attacking the Jedi Temple when the real culprit was Barriss Offee, who did it to show the Jedi that they've lost their way. Since the Council didn't trust her she can no longer trust herself, so she declines Anakin's offer of rejoining them and leaves to find her own path.
Twilight Sparkle in The Return of Harmony, when Discord's successful transformations of her friends into complete jerkasses causes her to lose faith in the power of friendship. Fortunately, she is quickly pulled out of this.
In Twilight's Kingdom Part 2, Discord after Tirek double-crosses him. While trapped in their bubbles, the Mane Five are frantic and begging Twilight to not surrender her magic for them. Discord, however, sits still, silent and slumped over, likely reeling with regret and seeming to have just given up. Traitor or not, the poor guy looks broken.
Manolo and Maria each get theirs when the other dies. Manolo lets Xibalba kill him in order to see Maria again, while she loses the will to oppose her father's plan to marry her to Joaquin.
Joaquin himself seems pretty close to one after Manolo dies; his best friend is dead and the woman he loves is only marrying him because her father pushed her into it and the man she really loves is dead.