Some heroes and heroines can take a huge amount of trouble and danger in their stride. Big Bad out to get them? Yesterday's news. Death Traps to the left and right of them? Snore. The End of the World as We Know It? They already know, thanks, and they're working on it. These characters know that a burden rests on their shoulders and don't let it get them down.
There's only one thing that can bring a seemingly invincible character to their knees: Betrayal.
Having the entire evil empire out to get them isn't a big deal as long as they have their True Companions watching their backs. If one of the chosen few should renege the hero will go to pieces.
It doesn't have to be anything as dramatic as switching sides or selling out to the Big Bad. If one makes a few scathing comments about their leadership skills, or another holds the hero responsible for his Cynicism Catalyst, the results can be as devastating as though they'd painted a target on their pal's head and handed out sniper rifles.
Can be played for comedy if their reaction is clearly an overreaction — surviving a cave in, an ambush, and a death trap without breaking a sweat, only to burst out into tears when his girlfriend forgets his birthday, for example. Generally it's anything but funny; most people can relate to having someone important let them down. Even if it's an overreaction it may reveal exactly how much stress the character is under.
A second version of this trope is where the hero has already given their buddy a second chance, overlooking past (or even current) misdeeds in the interest of preserving the friendship and deciding to accept their pal just the way they are, warts and all... as long as their misdemeanors are confined to people who aren't the hero or their immediate family. Should that unspoken truce be broken, a massive Freak Out is only seconds away.
A third version of this trope comes when the hero or the villain develops a bond with someone, only to discover that the person in question was never his friend, and just pretended to be one. Often followed by the obvious next question, Was It All a Lie?
This is, of course, Truth in Television — the vast majority of us care far more about what our parents, friends, and family think than what Joe Bloggs down the street may say about us.
Particularly brutal if the victim is betrayed by the ones who supposedly rescued him from a Friendless Background, or if they desperately seek friendship above all else. A Sadistic Choice, Break His Heart to Save Him or Because You Can Cope excuse may be made if the victim's "betrayer" felt as though they had no choice but to abandon them. If the characters in question were friends with each other prior to the betrayal, it probably triggers Face-Heel Turn, Evil Former Friend and We Used to Be Friends. Compare and contrast Plot-Mandated Friendship Failure, where the friend merely leaves rather than outright betrays him.
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Anime and Manga
In End of Evangelion, this is done twice (at least). In the past, when the first Dr. Naoko Akagi becomes the lover of Gendo Ikari, he instead callously betrays her by making a clone of his dead wife and makes it clear that she is not needed anymore. Heartbroken and enraged, she strangles the clone and then commits suicide by throwing herself from the control balcony. Years later, Dr. Ritsuko Akagi becomes another lover of Gendo, but she has rigged the MAGI computers to blow in hopes of killing Gendo in a Roaring Rampage of Revenge for both Akagis, only for her mother (in the programming of the MAGI system) to disable the bomb. Ritsuko is understandably devastated by this betrayal. Then, she is shot by Gendo. Yeesh, as if this show wasn't dark enough...
Tower of God - Rachel's betrayal of The Hero Baam is the pinnacle of season one. Her betrayal completely breaks him and she gets a free ride from his companions who think they are doing a favor in his legacy . This is also a reason why Rachel is a Walking Spoiler, since much about her is kept secret until that point.
Sasuke Uchiha of Naruto has reduced many a protagonist to tears with his willingness to kill his True Companions in cold blood, including the eponymous one, who starts bawling like a little kid when he realized that his former best friend-slash-substitute-brother is serious about killing him. A special mention goes to him telling a very useful, deeply loyal teammate (who was also in love with him) who had just been taken hostage not to move - naturally, her expression relaxes into a blissful smile... and then he stabs her along with her captor.
Sasuke himself has it when he learned that the Hidden Leaf Village is truly responsible for the Uchiha Clan Massacre, giving him a reason of wanting to destroy it.
Dragon Ball Z: Nappa was willing to fight the much stronger Goku, until, of course Vegeta betrayed him, and he was unable to do anything to defend himself.
This happens to Trunks and Goten; they were more than willing to fight Buu alongside Vegeta until he (apparently) offed Trunks (who was briefly shocked by his dad doing this to him), after which Goten entered a very brief Heroic BSOD after this before Vegeta took him out as well. Obviously, this was due to Vegeta needing to be alone to sacrifice himself against the monster he brought into this world.
Magical Project S: Sammy the magical girl had no problem beating the monster of the week until she discovered her worst enemy was her best friend Pixy Misa; she couldn't believe it. The same happened to Misao (aka Pixy Misa); she ran away from her home and her school when she believed that she had betrayed her best friend.
Kaleido Star's Sora puts up with a hell of a lot of grief in the second season — not that the first season was a picnic, but still. At various points in the storyline, she is demoted, fired, paired up with a partner who verges on sociopathic and spends the whole series struggling to earn the title of Kaleido Star — a title which she had won fair and square by the end of the first season, only to be usurped by a newcomer who wields far more power than any newcomer to the troupe has a right to. Through most of it, she remains perky and cheerful, convinced that everything will work out all right in the end...but when Layla, Sora's idol, suddenly turns on her and sides with Sora's rival, the poor girl finally cracks.
In Pandora Hearts, Jack is forced to kill his best friend Glen at the Tragedy of Sablier because Glen allegedly snapped and ordered the massacre of every man, woman, and child at the Baskervilles' mansion. In the biggest Wham Episode of a series already known for its HSQ, however, it's later revealed that Jack himself was the one who betrayed (Oswald) Glen's trust by opening the doors to the Abyss and attempting to bring the entire world down into it so that he could meet Lacie again. Ironically enough, just the night before Jack's betrayal, Glen asked his servant Gilbert whether or not he trusted Jack, and when the latter replied that he did, Glen decided to do the same.
Oz is betrayed by Jack when it's revealed that Jack was using him all along and actually considers him to be subhuman. Arthur Barma also applies and is of the friendless variety, as he was betrayed by Jack as well. Jack only befriended him so that he could later force him to write an account of the events of the Tragedy of Sablier that made Glen appear to be the villain and Jack the hero.
Hot Shot manages to hold off the bulk of Megatron's Decepticons in Transformers Armada (albeit while wielding the Star Saber). However, when Sideways betrays him and defects to the Decepticons, he falls to his knees and passively lets the Decepticons beat him into a metallic pulp.
In Berserk, Griffith faces all sorts of challenges to his dream without flinching, but when his best friend, the protagonist Guts, leaves him, he throws his dreams away with a stupid mistake. A year later, after being broken in both body and spirit, he clings to his humanity and even his hope...until he finds out that Guts and his other friend, Casca, are in a relationship and are discussing the necessity of him leaving the Hawks again.Then he snaps, resulting in an epic Face-Heel Turn as he makes a Deal with the Devil to become the fifth member of the Godhand and sacrifices the Band of the Hawks, saving Guts and Cascafor last.
Pet Shop of Horrors demonstrates the second type of Et Tu, Brute?. Leon is convinced that Count D is a serial killer by proxy. Yet the two form a fairly intense friendship regardless, with Leon spending most, if not all, of his spare time with D. Despite the constant banter and Leon's empty threats of arrest, the detective is clearly fond of D, and while he can't disregard the Pet Shop's body count, he doesn't let it disrupt their personal relationship. What does cause the breakdown in their association seems relatively minor by comparison to murder, but the intensely personal nature of the betrayal makes the difference: D sends Leon's little brother Chris, who D and Leon have been looking after for the past year, back to his adoptive parents (actually his aunt and uncle) without telling Leon what's happening. Moreover, Leon is distressed by D's apparent lack of regret or sorrow over losing Chris.
Arguably, it's D's reaction (or lack thereof), to the above event that constitutes a "betrayal" to Leon: for one, D doesn't send Chris back, Chris decides to go by his own free will, and two, since Chris is talking again, he couldn't stay in the petshop even if he wanted to. This is emphasized in the end of that chapter, and even earlier in the story. What really gets to Leon is D being (acting?) completely indifferent over witnessing Monica's death and distancing himself from Leon, treating him and, as Leon feels, Chris too, coldly and indifferently, as he did when they first met.
A variation occurs in Code Geass. About 2/3 of the way through the first season, Lelouch learns that his best friend Suzaku is the person who's been constantly wrecking his plans. Rather than feeling betrayed, he simply redoubles his attempts to recruit Suzaku for La Résistance. At almost the exact same point in the second season, after lots of misunderstanding and misdirected rage, the two are close to patching up their friendship...when soldiers rush in and arrest Lelouch for being a terrorist. Lelouch thinks Suzaku betrayed him (he didn't, he was followed), but it pretty well marks the absolute death of their friendship and the moment Lelouch stops showing Suzaku any form of respect or mercy.
Then after being exposed to Lelouch's Broken Pedestal, Suzaku quickly becomes a Knight of Zero, to be specific, the Knight above Knight of Rounds and The Dragon to the Emperor Lelouch. So...
Also, Suzaku has it on his side when he found out Lelouch has been opposing him as Zero, effectively ruining his plans to defeat Britannia by working with it so that he can change it from the inside.
If Kazuki has any plot relevance in a particular arc of Get Backers, this will happen.
Dio from Last Exile despises and fears his sister, but he can handle facing her as long as he has Luciola on his side. When he believes (possibly correctly) that Luciola has betrayed him to her, he snaps and hits him across the face.
Yomi from Ga-Rei Zero-. Short story: she killed a fellow exorcist who is also a fellow clan member, in defense and in a partially-justified rage. She expected her fiancee and her little sister to come to her defense, since she lost her capability of speaking after that battle. However, her fiancee never comes for her due to family honor. The final straw was her coming to a conclusion that even her little sister believes that she's guilty, and she went the Griffith's way.
It should be noted that said fiancee was working himself ragged(to the exclusion of his other, extremely important work) trying to prove her innocence so she wouldn't be cast out, and making preparations to take care of her in her disabled state, regardless of the outcome of his investigation. So even though she thinks he abandoned her... he's the one who was there for her the most. Which only makes it more tragic.
Light himself seems to react this way a little bit when Matsuda attacks him at the warehouse. "Matsuda, you idiot! Do you know who you're shooting at?" A unique example since Matsuda is reacting to Light's own betrayal when this happens.
In StrikerS Sound Stage X, Teana is rather badly shaken by the discovery that Runessa Magnus, whom she had come to trust enough to offer her a permanent partnership, is the mastermind behind their current case.
In Vampire Knight, Zero has this feeling when he found out Yuki is a Pureblood vampire all along.
In the comic series Preacher, Jesse quickly forms a best buddies relationship with Irish vampire Cassidy, travelling across half the world to save him at one point. But it's not long before Cassidy is trying it on with his girlfriend and later, after Jesse's apparent death, Cassidy manages to seduce her, getting her hooked on drugs in the process. When Jesse comes back (after learning of a long line of similar screw ups in Cassidy's past), he's not exactly pleased.
There's a scene when Jesse comes back, sees his girlfriend kissing Cassidy, and faints.
Basic premise of Avengers Disassembled. The Avengers were facing many threats, villains, and betrayals. But one of their core members, somebody they deeply trusted, suddenly snapping and trying to kill them, with clarification she's Not Brainwashed, was enough to disband the team.
In one Batman comic, Two-Face was genuinely reformed and about to leave Arkham. Then the Joker came along and started messing with him that his best friend Bruce Wayne and his girlfriend were together. As a final push, the Joker slipped an already suspicious Two-Face a fake newspaper that Bruce and his girlfriend were marrying. He broke out of Arkham to kill Bruce.
Graduate Meeting Of Mutual Killing: In the Chapter 3 trial, Reiji Ohmoto is accused of the murder. His brother Yukio defended him to the very end, and doesn't take it well when it's confirmed and revealed that Reiji really was the mastermind behind the murder and purposelly used Yukio as a pawn to get himself free
In Cars 2, Lightning McQueen gets this after his best friend Mater causes him to lose a race in the World Grand Prix.
A case where the betrayal actually does cause a Face-Heel Turn: Kung Fu Panda, during Tai Lung's Start of Darkness. Master Shifu betrays his prize pupil and adopted son—not by denying him the Dragon Scroll, but by raising him to believe he was destined to one day earn it as a matter of course, filling him with pride and dreams...and then, when Oogway advised otherwise, refusing to defend, stand up for, or even support his son. No wonder the snow leopard went insane and goes on his Rage Against the Mentor.
In the third movie of the Shrek series, Princess Fiona and the other Princesses' betrayal by Rapunzel, who was in love with Prince Charming.
Happens in Frozen when Anna is taken back to Hans, her fiance, so that they can share true love's kiss to melt her frozen heart. Just as they were about to kiss, Hans reveals his true intentions; he never loved Anna. It was all an act to get closer to the throne of Arendelle.
In Hercules, Meg is working for Hades, although not evil. When Phil tried to show this to the titular character instead of listening on her conversation with the villain to see if she will go through with his plan or not, Hercules doesn't believe him until Hades ended up revealing it to him, putting him into a Heroic BSOD from it.
Film — Live Action
When the protagonist of Braveheart learns that King Edward's personal bodyguard is none other than Robert the Bruce, he is so devastated that he sinks to his knees and can't move at all - and nearly gets captured because of this.
In the film Red Dawn (1984), the Wolverines seem to have no problem performing guerilla warfare against the occupying Soviet forces, gunning down countless Soviet soldiers. However, it's only when one of their members betrays them to the Soviets (he was captured, and they forced him to betray his buddies, otherwise they'd kill his father), and they are forced to kill him, that they realize how deep in the war they are in. After that, things progressively go downhill for the Wolverines.
TRON: Legacy : Flynn Sr. is pretty much broken already, but then he gets a good, solid look at the distinctive Tron Lines on Rinzler and realizes...
A relatively mild case where the betrayal actually does leads to the Fallen Hero's Face-Heel Turn in Skyfall. During the beginning of the film, M asks for James Bond to leave Tiago Rodriguez/Raoul Silva where he was, not only because he had been engaging in unauthorized hacking against the Chinese, but to hand him over to them in exchange for the prisoners held by the Chinese government. No wonder the guy becomes a cyber-terrorist bent on revenge against M.
In Trading Places, Louis Winthorpe is shocked when he's informed his former employers, the Duke Brothers, were responsible for his downfall because of a bet.
Louis Winthorpe: The Dukes ruined my life over a bet? For how much?
Billy Ray Valentine:A dollar.
Winthorpe:One dollar? All right... That's the way they want it? No problem.
(cut to Winthorpe loading several rifles)
In the Gone series, Sam and Quinn in the first book.
In The Bible, Judas betrays Jesus. The Jews that once supported Jesus now fervently denounce and mock him, calling for his crucifixion. It's subverted in that Jesus knew about the Judas's betrayal ahead of time, and, according to the Gospel of Judas may have even asked Judas to betray him.
In Jim Butcher's The Dresden Files novel Turn Coat, Morgan survives just fine being on the run for his life after being framed for murder. But let him learn the one person he cares for most thought the worst of him...
Though the person he perceives betrayed him gently points out Brainwashed and Crazy is a very real danger in this universe, and so the mistrust is not entirely unwarranted.
This is one of the worst ways you can hurt Harry. It's happened to him twice in his life, with Justin and Elaine before the series starts and Ebenezar in Blood Rites. Possibly the only way to hurt him more is to manipulate him into doing this to his friends.
This was the tragedy of the Marauders in Harry Potter, as explained in Prisoner of Azkaban. Peter Pettigrew, one of the four iconic best friends, betrays James Potter to Voldemort and sent him to his death, after James' best friend Sirius Black entrusted Peter with the secret of the Potters' hiding place. Sirius went to pieces upon discovering Peter's betrayal. And after Peter, rather than owning up to his betrayal when Sirius chased him down in a grief-stricken fury, framed Sirius for the Potters' murder and Peter's own killing spree, Sirius was so heartbroken that he broke down into hysterical laughter and was dragged off to Azkaban without struggling.
This trope is also heavily toyed with in Half-Blood Prince when Severus Snape kills Albus Dumbledore.
Animorphs got into this when Cassie let Tom get away with the morphing cube.
Bertie Wooster's reaction whenever he feels he's been betrayed or let down by Jeeves. It even merits a Julius CaesarShout-Out in "Jeeves and the Old-School Chum". (For the record, Jeeves had taken their lunch out of the car before they went golfing.)
I quivered like an aspen. I stared at the man. Aghast. Shocked to the core.
"You, Jeeves?" I said, and I should rather think Caesar spoke in the same sort of voice on finding Brutus puncturing him with the sharp instrument.
Song at Dawn Al-Hisba betrays Dragonetz by burning down the paper mill they created together. However, he only did it because the Archbishop twisted his arm and had attempted Dragonetz's murder three times thus far.
A Song of Ice and Fire: The Wall's commander Jon Snow gets knifed in the gut and back by several of his fellow Black Brothers, who believe that the changes he's made to the Wall are going to doom it. These changes include letting almost all the Wildlings into their ranks, being about to spend men on a borderline-suicidal mission to retrieve landlocked Wildlings, listening to Wildlings before taking the advice of fellow Black Brothers , and (though there are hints that they were going to do it even before this was put forth) announcing that he was leaving the Wall in order to go after a man who was causing trouble and threatening him. These are all Big Deals to anyone who knows the Wall's history.
Infernal Devices: Nate. What's with the constant brother/sister betrayals in Cassandra Clare books?
In the Left Behind book Assassins, Enigma Babylon leader Peter Mathews was killed in this manner by the ten subpotentates when he was invited to attend a Nasty Party where he was killed with Feather Flechettes from an ice sculpture made in his image. As it turns out, Peter's death was foretold by Scripture in the Book of Revelation, speaking of the "ten horns of the beast" that would hate the whore, that would "eat her flesh and burn her with fire", as Peter's body was then cremated under the cover story that he contracted a contagious disease.
Jonathan Stonagal, The Chess Master behind the creation of Nicolae Carpathia, also gets this from his own creation when he and Joshua Todd-Cothran were simultaneously murdered by Carpathia during the appointment of the ten regional subpotentates.
Live Action TV
The new Battlestar Galactica has some examples. These include: Starbuck's infidelity between Lee and Samuel Anders(Lee also does the same to his wife during this), Gaeta's Start of Darkness and participation in a coup, Ellen Tigh's infidelity, Saul Tigh's murder of his wife, Baltar's desertion of his cult, and Cavil's actions towards anyone who disagrees with him.
One Star Trek: The Next Generation had an abandoned Romulan prison camp full of Klingons, in which the two cultures merged and interbred. When Worf arrives and tries to teach the half-breed children about Klingon honor, the former Romulan prison commander sets up to execute him, and is standing there with his disruptor trained on Worf and all of the children go over and interpose themselves. He looks like he is about to pull the trigger anyway, but then his half-Klingon daughter (who has a crush on Worf) walks over to join them and utterly crushes his resolve.
A better Star Trek example comes from Deep Space Nine when a recurring character reveals himself to have been a mole for the terrorist/freedom-fighters the Maquis. Although the Maquis in general are depicted as a moral grey zone for the show (their cause is sympathetic but their methods illegal), Cpt. Sisko takes it VERY personally. The former Mole even lampshades this trope in a later episode after the Maquis have been all but eliminated, pointing out to Sisko that it wasn't their agenda or even their methods that pissed off Sisko so much, it was the BETRAYAL by those Masquis who had once been Star Fleet officers, turning their backs on the Federation.
Ironically, that same mole had previously betrayed Sisko by sabotaging the Defiant to stop Sisko from pursuing a ship...on orders from Starfleet.
There's an even clearer example earlier on, where Sisko discovers an old friend and colleague to have switched sides. He's known the man for decades, and has always seen him as someone who shares his values. The only difference between the two is that his colleague has experienced the problems that led to the foundation of the Maquis first hand.
In the episode "Inquisition," Dr. Bashir is hurt when even Sisko suspects him of suppressing memories of betraying the Federation. It is later revealed that everything that happened after a certain point in that episode was fake, including Sisko's reaction.
In Buffy the Vampire Slayer: Amy, Angel(us), Spike (occasionally), Faith, Billy Fordham, Willow, Gwendolyn Post and the list goes on. Even Xander may belong here, depending on how you interpret Xander's Lie.
Let's not forget Giles during the 'crucisomethingorothermentum'. Although, he does condemn it even before he does it, he was Just Following Orders.
Wesley, on the Angel side, betraying Angel and taking Connor.
In Merlin, Morgana leads Uther on to believe she's close to him, although they argue at times, and then she betrays him and seizes the throne. Ditto could be said for her and Arthur.
Morgana herself reacts like this when Merlin poisons her to save Camelot. To be fair, at the time he didn't have any other choice and he knew she had already betrayed them.
Firefly: The episode "Ariel" has Mal angry at Jayne because he feels personally betrayed by Jayne who called the Feds on Simon and River while they were doing a job. This shouldn't come as much of a surprise to viewers, who have seen Jayne betray/defect in the past. Especially in light of the pilot episode, where Jayne says it'll "be an interesting day" when the money is good enough for him to sell Mal out.
Mal: Next time you stab me in the back, have the guts to do it to my face.
In the White Collar episode 'The Dentist of Detroit' Mozzie is in danger from the Detroit Mob, and when Peter and Neal try to help him, Neal is forced to 'betray' him by giving him up to Peter as the Dentist of Detroit, to which Mozzie responds 'Et tu, Neal?' in a facsimile of this trope.
Brutus' betrayal and assassination of Caesar is recreated in Rome without the actual sentence being pronounced. Ciaran Hinds' facial expression, especially his eyes, carries the question "Et tu, Brute?" silently. Very effective.
In the Ashes to Ashes episode "Traitor", it is discovered that DC Chris Skelton has been feeding information to corrupt cops for the majority of season 2. This is the only time Gene Hunt ever becomes teary eyed and he is sent into a BSOD.
This is especially heartbreaking given that the traitor is one of the most timid and innocent cops in the station and has previously turned down any and all temptations of police corruption.
Inverted on Xena: Warrior Princess with Caesar's assassination. In Shakespeare's Julius Caesar, Caesar wilts upon seeing his close friend Brutus among his killers. But in Xena, Caesar had been cast as a much younger man(instead of middle-aged like the real Caesar was), and his response to realizing what was about to happen was an angry "And you, Brutus!"
Babylon 5: Michael Garibaldi winds up on both ends of this before it's all over.
At the end of the first season, Garibaldi gets wind of a conspiracy to kill Earth Alliance president, but before he can do anything he's shot In the Back by his own second, Jack. After he recovers and it's revealed what happened, he goes to Jack's cell, demanding to know why, then promising to be the one to push the button at his sentencing.
In season four, Garibaldi is under More Than Mind Control which eventually leads to him turning Sheridan over to the corrupt Earth Force. After this, Bester comes and releases the mental programming, leaving Garibaldi in his right mind again, and fully aware of what he's done. He first tries to get in contact with his former allies, but they're not exactly sympathetic, and he has to beg Lyta to scan his memories to prove his story.
Lifetime has an entire series entitled "I Killed My BFF."
Dungeons & Dragons 4th edition mentions in the Codex Of Betrayal articles, which chronicle the origin of devils and their war against the god known only as He Who Was. It's said that when Asmodeus, leader of the rebellion, convinced Greyon, HWW's favorite angel, to side against him, the god cried such tears as to drown mountains, and after such a betrayal, his heart was no longer in the war.
Older Than Steam: The trope name comes from William Shakespeare's Julius Caesar. As the assassins attack, Caesar defends himself... but when he sees Brutus, his best friend, among the assassins, he gives up and lets himself be murdered - he didn't care about a bunch of strangers armed with pointy things, but having his buddy stab him is another story entirely. The full quote is: "Et tu, Brute?note Even you, O Brutus? Then fall, Caesar." Marc Antony during Caesar's funeral would say of Brutus's betrayal that his was "the most unkindest cut of all."
One alternate interpretation of the line is less of a question and more of a statement, as in "You're next." In real life Brutus only lived another two years after the death of Caesar.
Max Bialystok calls Leo this in The Producers when he receives a postcard from Brazil and learns what happened to Ula and Leo along with other infamous traitors in history. Both come back and Max reconciles with Leo.
Fallout: New Vegas: Mr. House essentially invokes this if you kill him. Caesar as well if you had sided with him and then kill him in the aptly-named quest "Et Tumor, Brute?"
Final Fantasy XIII Sazh had no problem fighting with Vanille against PSICOM until he realized that Vanille was the reason his son became a l'Cie he was about to kill her and commit suicide. He fortunately came back to his senses.
Halo The Arbiter fought against the flood and humanity with no problem at all but when he realized that the Prophets (who gave him a second chance as The Arbiter) betrayed him along with all the ones of his kind, The Arbiter himself was betrayed by Tartarus, he couldn't accept the truth until The Gravemind with the help of Master Chief showed him the truth.
Video game example: The Suikoden series. A running theme through the games is the main hero having to deal with the betrayal of a close friend or ally — which he usually has the option of forgiving. (Doing so helps determine what ending the game gets.)
The Mass Effect series has plenty of examples, beginning with the second death in the series: Sacrificial Lion Nihlus is shot in the back of the head by his old colleague Saren.
Garrus's whole loyalty mission is entirely focused on him finding and dealing with someone who did this to him. After Shepard recruits him, he tells him about a traitor named Sidonis who sold out his squad to the various merc groups on Omega and told them where the squad were hiding, causing the mercs to track them down and kill everyone but Garrus. You find out later that this wasn't quite the case, as the mercs actually threatened him at gunpoint for the location of the base, and he feels very guilty for actually giving in to their demands. While Garrus at first still wants to execute him for being a coward, you can convince him to back down.
This also happened to Zaeed Massani, courtesy of Vido Santiago. He and Vido founded the Blue Suns mercenary group as a business venture over twenty years ago. While mercenary groups aren't exactly known for being morally scrupulous, Zaeed tried to keep the Blue Suns fairly clean. Vido, objecting to this because it cut profits, turned Zaeed's men against him. He paid a group of them to hold him down while he shot Zaeed in the head (Zaeed survived).
Shepard has a Heroic BSOD when the Council pulls him/her off the hunt for Saren. For a Paragon Shepard, this comes as a real blow.
Shepard him/herself can do this in Mass Effect 3 for various reasons. A particularly bloodthirsty player can personally kill four close friends - Mordin, Wrex, Legion and the Virmire Survivor - in a single playthrough.
Fenris of Dragon Age II is a former Tevinter slave on the run from his master, Denarius, who is... not a nice man. He catches up to Fenris in the last third of the game, while Fenris is in Hawke's company, and Hawke has the option of fighting him off—or giving Fenris back to him. Fenris hits this reaction hard, especially if he and Hawke are friends, and especially if he's in a romance.
In ''Starcraft there tend to be a lot of instances of people betraying others. In SCII, Tychus reveals that he was given an order to kill the newly de-infected Kerrigan, regardless of the fact that she's no longer zerg, and as a result Raynor has to kill him.
In Mega Man X 7's prologue, when Axl and Red Alert were infiltrating the base of a maverick. His second in command seemingly finds him, and then while leading him away, he shot the guy in the back. However, in this case, it was a subversion, as the actual second in command had actually been killed earlier by Axl, and the "second in command" who shot his boss was actually Axl.
Cole Phelps in L.A. Noire has no problem working with Roy Earle, despite the fact they don't like each other until he discovers he's the one who reported his affair with Elsa Lichtmann to the corrupt officials.
Dante in Devil May Cry is hired by Trish to look for Mundus in an island. This is because she's working for him. When Dante found that out, he is not happy about this. He and Trish finally make up in the end, anyway.
A double-wammy in Arc the Lad: Twilight of the Spirits, Darc forms an alliance with Delma and her power-hungry brother Densimo. When his father's killers arrive, they give Densimo a furble, which he can eat to expand his power. Densimo takes the offering without hesitation. Darc is also betrayed by his master Geedo, who treated him like crap but he still loved her for caring for him. When he realized how little he meant to both, he went off the deep end and killed both of them as well as the murderers, and vowed to become king of the Deimos.
Connor from Assassin's Creed III is none too pleased when he found out George Washington is the culprit in the burning of his village, and also the fact that his father, Haytham, has been hiding this information from him that he cuts his ties with them both.
Tohsaka to Archer in Fate/stay night's Unlimited Blade Works route. After keeping calm throughout the story, she's devastated and doesn't hide it terribly well when he betrays her twice. On the plus side, the event is a catalyst for her Relationship Upgrade with Shirou. It's also a real bonding experience for her with Lancer.
Hell, this trope is the only reason Saber exists. Although she fought virtually the entire world as King of England, and never lost a single battle for 12 years, she died at the hands of her own trusted men. As well as Archer, Who ended up getting betrayed and killed by the very people he dedicated his life to protect.
In the science fiction visual novelBionic Heart, the protagonist ends up hiding a fugitive android in his apartment because she needs his help in her ontological quest. If the main character chooses to confide in his best friend and coworker Tom, Tom ends up giving up both the protagonist and the android to the police, which leaves them in the hands of the corrupt corporate executives that were pursuing the android in the first place.
Shizune's bad ending in Katawa Shoujo. Her boyfriend cheats on her with her best friend. The resulting guilt causes their relationship to spiral out of control until she breaks it up- and she blames herself.
In the webcomic MegaTokyo, Largo himself points out, in an unusually serious tone, how painful a betrayal can be. In fact much of his dislike for Miho, previously played for laughs, stems from her once being their friend online in an MMORPG gameverse (and seducing Piro's character) before viciously betraying them. This is hinted as part of the reason he is so mistrustful of relationships, something Erika brands as fairly laughable at first before realising how seriously he took it.
It's interesting to note that it seems to have been played for laughs as well as (some) drama, instead of one or the other, but that's to be expected of LFG in general and Richard in particular.
The real Et Tu, Brute? moment is when Cale learns that Pella killed their own soldiers to force Cale to retreat and therefore survive. He knows that his allies aren't the most moral of people, but he thought that she was more idealistic like him.
The Nostalgia Critic has done this in both That Guy with the Glasses anniversary specials: in the brawl to 2D Lee because he sided with the gamers over the critics, and in Kickassia to Film Brain when he turned against Critic because he accidentally killed Santa Christ.
Chakona Space: In chapter 7 of Tales of the Folly, Neal has been injured in some spaceport drama. Not only are his mates, companions and children trying to tie him down in a sickbay bed, so is his ship. He then proceeds to quote Julius Caesar.
In Worm, Rachel Lindt, a supervillain, takes it badly when it's revealed that Taylor, quite possibly the only friend that she's ever had, is an aspiring superhero who joined Rachel's gang with the intent to betray them to the authorities. Their friendship afterwards is strained to the point of breaking.
As Told by Ginger: When Ginger Foutley finds out from Courtney Gripling that Dodie Bishop and Macie Lightfoot teamed up with Miranda Killgallenin an attempt to break up her and Darren Patterson up.
Lampshaded when Ginger sees her friends defecting to the other side in "Battle Of The Bands".
"Et tu Dodie?"
When Terra betrays the team in Teen Titans, she is easily able to hunt down and defeat them one by one, because they're unable to fight a former friend using their full strength (save Raven, whom she actually provokes into doing so, which nearly backfires on her). When they do finally let loose, Terra goes down in minutes.
Villainous example in Avatar: The Last Airbender : In "The Boiling Rock", Azula's spiral down into complete insanity starts when Mai and Ty Lee betray her. Arguably, they were never her friends to begin with; they were just scared of her.
In the episode, "The Avatar and the Fire Lord", though the relationship between Avatar Roku and Fire Lord Sozin became deeply strained when Roku made it clear that he would kill Sozin if he launched his planned war, they were still friends, as shown when Sozin showed up to aid Roku when his island suffered a massive volcanic eruption. The two men stood their ground and controlled the volcanoes until the villagers had escaped, at which point Sozin realized Roku was vulnerable and betrayed him, leaving him to die on the island so he could fulfill his ambitions of world conquest.
In the episode "The Crossroads Of Destiny", Uncle Iroh's betrayal by Zuko, his nephew who sold him out to the Dai Li.
A rather depressing example comes from Batman: The Animated Series. In the episode, "Birds of a Feather", the Penguin is released from prison and declares that he's reformed and will become a model member of high society. A group of snobbish aristocrats decide to bring him into their social circle so that they can laugh at his social ineptitude and appearance. He generally doesn't care how life had gotten him down through the rest of the episode, but when he overhears the woman whom he had fallen in love with talking about this plot, he loses it. The real slap in the face is that he had genuinely reformed until this happened.
The Beetlejuice cartoon has an instance. In the episode, "Mr. Beetlejuice Goes to Town", after Beetlejuice gets elected Mayor of the Neitherworld, he lets the success go to him. Lydia goes incognito as a special interest rep and bribes Beetlejuice into some questionable legislation that gets exposed. After being impeached, B.J. has an Et Tu when he learns that Lydia was behind his impeachment.
In the episode, "Injustice For All", Lex Luthor was hit in his armor by Humanite with a "killswitch" device when the latter sneaks up behind him. His words being, "Et tu, Humanite?", before he went unconscious.
A comedic example happens in the episode, "Party Of One". Pinkie Pie assumes that her friends are turning their backs on her after Spike said that they don't like her parties anymore. As it turns out, her friends are setting up a birthday party for her without telling her about it.
A brainwashed example occurs in the two-parter episode, "The Return Of Harmony". In that episode, Discord, knowing about The Power of Friendship, brainwashes the True Companions into turning against each other. After a Circle of Shame by the Discord's balloons, which tells her that her friends are laughing about her behind her back, corrupted Pinkie Pie's reply to Twilight Sparkle is to be expected when she and the corrupted Applejack found her:
Twilight Sparkle: Pinkie Pie! Are we glad to see you! Pinkie Pie: Oh you are, huh? Why? Need a good laugh?
To say nothing about how poor Twilight felt when one by one, her friends are acting different from themselves, and when Rainbow Dash went flying out of the maze after what Applejack told her.
Applejack: (watching as Rainbow Dash went flying out of the maze) Well, looky there. Rainbow Dash is flying away. She's abandonin' us. Twilight Sparkle: Now I know that's a lie. How can it be?
A relatively mild example occurs in the episode "Ponyville Confidential". When the Gabby Gums column begins to print false-but-still-damaging stories of the mane six, it's initially suggested to Rarity that since Sweetie Belle works for the school paper, she might know who GG is. Rarity is immediately offended by the implication that her sister would associate with a pony with such blatant disregard for everyponys feelings. Her tune changes immediately once she discovers Sweetie Belle stole Rarity's diary and published it. To drive this home, Rarity confronts Sweetie with "Et tu, Gabby Gums?"
Before her, Shining Armor is likewise stunned that Discord would turn his back on them.
Discord gets hit with this moments later by Tirek, who he seemed to have thought was actually his friend. It causes Discord to, for the first time in the series, be broken.
In the Grand Finale of Superman: The Animated Series, Superman is on the run due to attacking Earth while being Brainwashed and Crazy, and Supergirl is badly wounded and needs medical treatment. When he seeks the help of his scientist friend Emil Hamilton who has previously always stood by his side, Emil instead refuses to help because he doesn't want to consort with someone who's a known fugitive. Supes gets so enraged his "friend" is pretty much throwing him to the dogs he winds up threatening to kill Emil unless he helps Supergirl. The Man of Steel quickly winds up being horrified at his actions and softens up, but Emil in turn wins up getting his own Et Tu, Brute moment from this, leading him to switch sides and work with Lex Luthor in Justice League Unlimited.
Scooby-Doo! Mystery Incorporated: Marcie Fleach's final conversation with her former employer Mr. E before she's executed, plays out this way, as all his efforts to stop the evil have come to ruin, and he's going along with the attempt to free it.
Marcie: "E I guess I expected more from you."
Mr. E: "So did I little girl, so did I."
The New Adventures of Winnie the Pooh: In "Rabbit Marks The Spot", when Rabbit tries to teach the others a lesson in digging in his garden for treasure by filling up a chest with rocks, he has a nightmare in which his friends, as stone statues, call him out for pulling such a stunt on them. Upon awaking from his nightmare, he fears that he will be hated forever if they found out what's in the chest. Of course, this is long before it's revealed they actually appreciate what kind of worth the "treasure" has to offer.
In The Venture Bros., Monarch says this to King Gorilla when the latter (initially) refuses to help the Monarch escape prison.
Civil wars are often bloodier than "regular" wars because of this trope.
According to Roman historian Suetonius, the realJulius Caesar said something similar, but it was actually in Greek: kai su, teknon? means "You too, my son?" Other accounts suggest that Caesar said nothing at all, but he pulled his toga over his head when he saw Brutus among the conspirators.