The Beast in Beauty and the Beast gets hit by this trope hard. When he lets Belle go with mere hours left to break the curse, he knows he has lost the love of his life and permanently destroyed any chance of becoming human again. He is so distraught that, when Mrs. Potts comes to inform him that Gaston and his cronies are invading the castle, he glumly replies "let them come". When Gaston reaches the Beast's tower, he doesn't even TRY to fight him at first.
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.
When a llama transformed Kuzco from The Emperor's New Groove learns that Yzma and Kronk are trying to kill him and Pacha has abandoned him, he gives up hope that hell ever turn back into a human. He then tries to start a new life as a llama.
After spending 13 years teetering on the edge, Elsa from Frozen fully crosses it when she is informed by Hans that her sister is dead because of her own actions, her very own fear coming true which makes her numb so much that she falls to the ground, and makes no acknowledgement of anything else that happens around her, either because she's too lost in her grief to notice or because she no longer cares.
You are lost Hope is gone But you must go on And do the next right thing
Bob has one in The Incredibles: When his entire family is believed to have been killed by Syndrome's missiles, Bob completely breaks down in a heartbroken but also murderous rage, threatening to kill Mirage but is brushed off by Syndrome. Later on, after Mirage releases him, he nearly chokes her to death in a Neck Lift and asks in an extremely chilling tone "Why are you here? How can you possibly bring me lower? What more can you take away from me?"
Abbot Cellach in The Secret of Kells when his obsessive efforts to protect his village from Viking attack backfire and nearly everyone is killed, seemingly including his young nephew and only remaining relative. At the end of the film, we move forward nearly twenty years and see he is still haunted by what happened that day.
Vanellope has one in Wreck-It Ralph, when Ralph destroys her cart, believing (from King Candy/Turbo) that she will die if she becomes a racer. It's later averted, however, when the cart is repaired by Fix-It Felix.
The soldiers in 28 Days Later, had apparently crashed over this line before the events of the movie had even taken place as they believe the entire world has ended and that humanity has no future. The only one to remain a noble and heroic man was Sergeant Farrell, and it's potentially only because he believes The Virus couldn't have escaped the island and that the rest of the world has simply quarantined them. He's right.
In 1408 Mike Enslin's child daughter is brought back to life just to die in his arms and THEN THE BODY CRUMBLES INTO ASH! You can tell he is losing it as he tries to put Katy's "pieces" back together then his face afterward is just a total emptiness inside, and the room keeps going.
In The Bay, Officer Jimson experiences this after performing mercy kills on a house full of people infected by isopods, then becoming infected himself.
Near the end of The Blair Witch Project, Heather is alone in the woods, her friends are missing, and she blames herself for what had happened because it was her idea to film the documentary in the first place and and her bull-headedness that got them lost in the woods with an evil witch's ghost. At the end of her rope and knowing that the witch is coming for her next, she looks into the camera and tearfully apologizes to her mother and Josh and Mark's mothers for getting them all killed.
Heather: I'm scared to close my eyes, I'm scared to open them... I'm gonna die out here!
In Braveheart, this trope hits William Wallace after he finds out Robert the Bruce has betrayed him. His anger immediately turns into distress and he seems to simply give up, which also later causes Robert to suffer from My God, What Have I Done?.
Happens to Harry in Burn After Reading after he accidentally kills Chad in the closet who he thought was a spy. Ever since, Harry developed a Hair-Trigger Temper and distant, obsessive behavior due to paranoia and the murder he committed.
In Cloverfield, right after the main character's brother dies on the bridge, you can see the exact moment that his mind breaks and self-preservation stops mattering.
In Cube 2: Hypercube, Sasha, aka Alex Trusk, eventually gives up hope that she can escape the hypercube and completely resigns herself to her impending death. This doesn't stop Simon from murdering her in cold blood anyway.
Dead Poets Society: Neil decides to pursue his dreams of performing onstage by starring in a local production of A Midsummer Night's Dream. Despite getting a standing ovation for his performance as Puck, his father still drags him out of the theater and takes him home, telling him he's getting pulled out of Welton Academy and sent to a military school, instead, so that he'll get ready for Harvard and medical school after that, and he's hellbent on not allowing him to deviate from that path even a little. Neil, heartbroken that his father refuses to listen to or understand him, kills himself, afterwards.
The death of Ernst-Robert Grawitz by Pater Familicide, as depicted in the movie, was another example of how the fall of Berlin to Soviet forces caused many a Despair Event Horizon for Germans in the war.
The whole film is basically a chain of these as every major character is broken in facing their inevitable, total, and deserved (or undeserved, in some rare cases) defeats. Hitler himself has about half-a-dozen moments where he crosses the line — while the man flits between episodes of psychotic rage and delusional optimism, he only really begins actively contemplating suicide when he learns that Himmler, the leader of his Praetorian Guard and the man Hitler believed to be his most loyal follower, is meeting with the Allies to negotiate surrender terms for Germany.
White being disqualified in Exam. Upon realizing that he was played by Deaf, he loses it and tries to kill himself with the Guard's gun.
In The Expendables 3, Barney goes through this when Caesar is nearly killed by Stonebanks. Barney is so distraught that he temporarily disbands the Expendables and forms a new team of young recruits because he can't bear the thought of one of his friends being killed so close to retirement.
The Fourth Wise Man: Artaban hits this when in the course of twelve hours everything he's worked on is destroyed, he is told his father died alone and surrounded by strangers, and an old rival mocks him for his idealism and foolish quest. Doesn't help he already knew he was dying.
In Full Metal Jacket, Private Gomer Pyle is driven into a psychotic breakdown both by the original Drill Sergeant Nasty, Gunnery Sergeant Hartman, and by the rest of his platoon, which ultimately leads to Hartman's murder and his own suicide. The moment when Pyle hit the Despair Event Horizon was probably when Hartman found a jelly doughnut in his footlocker just when things were starting to go well for him and proceeded to punish the entire platoon for it, which was then followed by the platoon taking it out on Pyle in the harrowing "blanket party" scene.
In The Grizzlies, Inuit student Zach is arrested for stealing to get money for his brother to escape his neglectful, alcoholic parents. As he has two priors, he realises he'll be sent south to juvenile hall, thus unable to care for his younger brother. He ends up hanging himself in his cell.
In the Fade: Katja suffers this twice. First, when her family is murdered. After the police fail to find their murderers, she decides to end her life. Then, while doing so, she's interrupted by a phone call telling her they were caught, so Katja saves herself. However, they are acquitted, so she gets this again. She then decides to simply kill them, along with herself, clearly not wanting to live anymore.
Joker (2019): The point where Arthur spirals out of control is when he finds his medical files, which tell him that he was not Thomas Wayne's son (as he'd been led to believe by his mother) and was abused for years (which he had no memory of due to the severity of the abuse he faced). At that point, he spirals down into increasingly violent behavior and kills both Penny and Randall, before planning to kill himself live on television. It's only after killing Murray on live televisionthat he has a new lease on life -- as the Joker. It's best exemplified in what Arthur says right before he decides to kill Murray:
Arthur: I've got nothing left to lose. Nothing can hurt me anymore. My life is nothing but a comedy.
In Lawrence of Arabia, the turning point of the movie is the capture, torture (and implied rape) of the protagonist by the Turks. The cocky, bemused Warrior Poet who believed to be invincible turns into a bitter, grim Anti-Hero after that.
In the Italian comedy Le Comiche 2, the lawyer character crosses this line once he learns he is going to receive an undesired male-to-female sex reassignment surgery. Earlier in the movie he already received a breast augmentation and his emasculation became a running gag. After endless physical and psychological abuse (Played for Laughs), finding himself on the operating table, he gives up and simply faints instead of trying to avert the surgery.
In Left for Dead, Preacher Man Mobius Lockhardt is completely shattered when he is forced to watch Mary Black and her gang of whores murder his wife and their unborn child in front of him. He renounces God and gives himself to Satan.
Let Me In: Owen crosses it near the end of the film. After seeing his girlfriend Abby, the only person who's ever been kind to him, kill a policeman in front of him and announce that she's going to have to leave. He looks like he's in shock and he watches her leave in a taxi without showing any emotion. The next day the realization that he's lost his only friend and he's going back to his old life of being neglected and abused by everyone around him catches up to him and he looks out at the jungle gym they played at, crying his heart out.
Lord of War: Pay attention to Valentine's face when Yuri makes his final speech. He gets closer and closer to this over the course of the speech and finally crosses when the door knocks.
Averted by Zod and Co. at the destruction of Krypton in Man of Steel. They're obviously crushed by its loss, but expected it and are still functioning. When the ship containing Krypton's only chance at resurrection is destroyed by Kal-El, and Zod is left kneeling in the literal ashes of his permanently dead people, he snaps and becomes a Death Seeker immediately.
The President of the United States in Mars Attacks! has apparently crossed this line by the time he finally gives in to his General's request to fight back against the Martians using nuclear weapons.
Thor: Odin's "No, Loki" has this effect on his son. Loki loses the will to go on, lets go of Gungnir he was holding to while hanging over a wormhole, and allows himself to be sucked into the void of space... making it a literal despair event horizon.
Iron Man 2: Downplayed, right before Tony's birthday party; Tony, suffering heavy metal poisoning from the miniature arc reactor in his chest, eventually succumbs to the idea that it no longer matters what he does with death so close. The problem is that he does it while wearing the suit in a room full of helpless people.
Star Lord, when he learns that Thanos has killed Gamora for the Soul Stone.
Tony, Steve and the other survivors are all distraught and look unable to go on anymore, when Thanos completes his Infinity gauntlet and snaps his fingers, wiping out half of the universe's population.
At the end of The Maze Runner, everyone back in The Glade died with Gally the sole survivor and stung by a Griever, he lost all hope and tries to kill everyone, including himself.
Happens to Zordon of all people in Mighty Morphin' Power Rangers: The Movie, after Ivan Ooze destroys the Command Center and the source of the Rangers' power. Though Alpha-5 gets the Rangers started on the hunt for a new power to fill the void, Zordon never really recovers, plainly stating that Ivan Ooze had won, and dies without ever finding out if the Rangers were successful or not. He is understandably very surprised when the Rangers manage to resurrect him at the end of the film.
In the Monty Python film Now for Something Completely Different: Parodied in the "Marriage Guidance Counsellor" sketch. At the end Mr Peuty, in despair because his wife is currently seducing the counselor, walks out of the office, whereupon a 16-ton weight drops on him and the caption reads "So much for pathos."
When Atreyu finds the Rock Biter in The Neverending Story, he finds that the Rock Biter's friends have been destroyed by the Nothing, despite the Rock Biter's best efforts. Now he can just sit and reflect how useless all his strength was and wait for The Nothing to overtake him.
Rock Biter: They look like such big, strong hands, don't they?
Our Miss Brooks: In the cinematicgrand finale. After overhearing a conversation at the relator's, Miss Brooks discovers that Mr. Boynton has bought the cottage across the street from Mrs. Davis' house. The conversation suggests that he finally intends to propose. Alas, he bought the house so his widowed mother could move in with him. This comes as a shock to Connie, who had even brought wallpaper over to the cottage to decorate. She's lost in daydreams, when Mr. Boynton comes in relates his plans to live with his mother.
Connie:(sobbing) Fine schnook I've been! (She hands the wall paper to Mr. Boynton) Wear it in good health! (Connie leaves the cottage, slamming the door behind her.)
Miss Brooks goes into a deep depression, offers her resignation and prepares to leave Madison. Fortunately, the matter is fixed by the good offices of Mrs. Davis and Mr. Boynton's mother. Mrs. Davis tells Mrs. Boynton the situation, and invites her to be her new boarder. Mr. Boynton proposes to Miss Brooks, and everybody lives Happily Ever After.
Red Dawn (1984). Things are going well for the American guerrillas until several of their group get killed trying to get a downed pilot across the front lines (the pilot also dies). Then one of their group turns out to be a traitor and has to be executed by his friends. A change in Soviet tactics leads to more of them getting killed in an ambush, so they're down to only four people. The leaders of the group, the Eckert brothers, decide to head into their Soviet-occupied hometown and go out in a Bolivian Army Ending, drawing troops into the town so the last remaining two can escape to the US lines.
Tony Montana crosses it in Scarface (1983) when he realizes that his impulsive protectiveness of his sister Gina caused him to kill his best friend. When Sosa's men kill Gina at the end, he proceeds to go into an Anti-Heroic RRoD that (at least in this film) ends in his death.
In the spirit of one-upmanship and outdoing the rest of this list, A Serbian Film has Milos have one after learning that he was drugged and made to rape and kill people, including his own son, he was raped himself, and all of it was filmed for a snuff director's entertainment after which we get a Shower of Angst shot with him in the fetal position in the shower. Eventually, this leads to him killing himself.
The main character in Signsgives up his belief in God entirely after his wife is killed in a car accident. He gets it back when the "nonsense" she was saying as she was dying turned out to be important clues.
In every version of Sleuth, the objective of the various games that Andrew and Milo play is to drive the other to this point. It starts with Andrew convincing Milo that he's about to be the victim of a perfect murder, and only gets more twisted from there.
In Speed, Jack almost crosses it when Harry dies in the raid at Payne's house.
In Return of the Jedi, the Emperor tries his best to push young Skywalker over this, so he will turn to the dark side, by showing the Death Star is still shielded, his friends are going to die, and the Rebels are going to be defeated once and for all.
He had reason to suspect it would work on Luke — it worked just as well on his father in Revenge of the Sith. Anakin struggles through the first half of the movie to remain true to the Jedi way while still protecting Padme. He goes to Yoda for assistance in understanding his dream of her death, and Yoda's only advice is to 'let go of what he fears to lose,' something he can't do. Palpatine dangles hope in front of him with the tale of Darth Plagueis, and Anakin's faith in the Jedi begins to waver, but when Palpatine admits he's Sith, Anakin still returns to the Jedi and tells them about him. When he finally breaks and disobeys the order to remain at the temple, he argues with Mace Windu about keeping Palpatine alive, first with 'he's a criminal and must stand trial' and then how he needs Palpatine alive. When he attacks Mace, he intentionally only cuts off Mace's hands, leaving him alive and capable of having them replaced. It's Palpatine who kills Mace with Force lightning, and at that moment, Anakin realizes that he's betrayed everything in his desire to save Padmé. It's all he has left at that point, and he gives himself over to the man who promises it. Which he doesn't. That Big "NO!" is probably the last moment of Anakin Skywalker until Luke reawakens him, twenty years later.
Padmé goes over this when she learns what Anakin has become, recognizing that the man she loved is effectively dead. Obi-Wan teeters perilously close to this for the same reason, but manages to pull himself out of it.
In The Force Awakens, it's explained that Luke himself suffered this when he restarted the Jedi Order, but it was destroyed when his student Kylo Ren aka his nephew Ben Solo turned to the Dark Side of the Force and wiped out his other students. Blaming himself over what happened, Luke soon went into hiding, totally severing contact with his friends and relatives, seemingly resigned to be the last Jedi once more.
The Last Jedi gives us some more details. Luke blames himself because he sensed Ben's dark side, and, in a moment of weakness, believed he could kill Ben and prevent more suffering in the galaxy. The moment passed, but seeing his master standing over him with a lightsaber drawn was all that was needed to push Ben over the edge. Tormented by his mistake, and disillusioned by the continued failures of the Jedi, Luke resolved to never teach another generation of Jedi, lest they turn to the dark side, or die due to his arrogance, going into exile and cutting himself off from the Force. He gets better by the end of the film.
In Still Alice, Alice goes through this as her Alzheimer's progresses, eventually culminating in her attempting to OD on a bottle of sleeping pills, only to be interrupted by her caretaker.
In Thirteen Women, June Raskob goes mad after being manipulated into killing her twin sister May by dropping her during their trapeze act. Grace later comments that June has been confined to an asylum ever since the accident.
Quoted on the main page: In Trainspotting, Sick Boy, while morally ambiguous, still has his good points. That changes when Baby Dawn, now revealed to be his daughter, dies.
Transformers: Age of Extinction proves that even Optimus Prime isnt safe from this. Betrayed by the humans and with most of his Autobots likely dead at their hands, and probably not helped by the fact that they unwittingly brought back his arch nemesis, he is fully prepared and willing to abandon them and leave them to their self-destructive fate.
The flashback scene in TRON: Legacy. Clu takes over, the Sea of Simulation is poisoned so no more life can come from it, Tron is thought dead, but it's muchworse, the Iso Cities are destroyed, and the portal back to the human world flickers out. The brash and cheerful protagonist for the first film clearly died at that point, leaving behind a hollowed-out Zen Survivor.
Wonder Woman (2017): Diana gets dangerously close to the precipice when Steve sacrifices himself. In her despondent rage, she vents on all the German soldiers, and comes close to gruesomely crushing Dr. Poison with a tank. She pulls herself back when she tries hard to figure out what Steve was saying to her when her Steel Eardrums failed her a few moments before.
In X-Men: Days of Future Past, when Xavier is forced to close his school after the first semester due to The Vietnam War conscription, he gives up his ambition to be a leader and protector of mutants, and becomes a self-medicating recluse.