Arashi Kishu in X1999. In order to spare her lover Sorata from his obligation to sacrifice himself for her, she joins the Dragons of Earth to bypass fate. She battles her former Dragons of Heaven friends and eventually triggers said prophecy with her Face Heel Turn.
In the manga, by all appearances, Arashi ends up Brainwashed and Crazy at the hands of Hinoto. With the same result, fighting for the Dragons of Earth.
If there's going to be an X1999 example, it ought to be Kamui and Fuma. They were best friends as children, and destined to be on opposite sides of the battle for the future of mankind. Whichever side Kamui chose, Fuma would automatically take the empty seat on the opposite side as a substitute "Kamui".
Ash Ketchum and Gary Oak of Pokémon fame. They were good friends and neighbors when they were very young. It's explained that they fought over a Poke Ball they found and broke it, and stopped being friends over it. It turns out each kept his half of the Poke Ball, and they are on better terms again.
Berserk - Guts and Griffith used to be the best of friends during the Golden Age arc. Then Guts left the Hawks in order to pursue his own dream, and Griffith's life went straight to hell, and by the time that Griffith was rescued, he all but despised Guts. Guts, however, still held on to his feelings for Griffith up until the Eclipse, where Griffith's monstrousbetrayal would destroy the bond of friendship between the two men forever.
Pretty much the whole point of Part 2 in Naruto. Sasuke Uchiha is the patron saint of this. Even his new allies who were ready to give their lives for him were quickly discarded.
What's interesting in this case is that Naruto's devotion to saving Sasuke - in one form or another - actually causes breaks between him and his friends, who think Sasuke is an evil back-stabbing bastard who needs to die. So essentially Sasuke has instigated a chain reaction of this trope.
Even before that, we have Ino Yamanaka and Sakura Haruno stop being friends because they are competing over Sasuke (who doesn't much care for either one of them). They become friends again after fighting each other (which, at least to Sakura, was more about proving herself equal to Ino, whom she had held in high regard ever since she met her, than about Sasuke), but do still quarrel over Sasuke from time to time.
Jiraiya and Tsunade with Orochimaru. Jiraiya confronts Orochimaru during his escape from Konoha, and reminds them that the Sannin were friends since their team was formed when they were five years old, but Orochimaru mocks his efforts.
Happens to Yomi and Kagura in Ga-Rei -Zero-, the prequel anime to the manga. Yomi's fall is pretty heartbreaking, and they only further make it more painful by the Big Bad who unleashes an attack on Yomi which doesn't kill her, but severs her voicebox and nerves so she's only barely able to communicate by using her finger, and can't speak. The scene where she's trying to cry in the hospital after her fiance is forced to break off their engagement is particularly heartbreaking.
Tower Of God: Baam and Rachel. In this case, it doesn't end well, and the falling out started before Baam even realized it: The moment he followed her into the Tower. In the end, she drowns him and he barely survives, crushed by the fact that his only family, friend and possible lover now hates him, where the only thing he truly feared was being alone.
Miaka and Yui in Fushigi Yuugi. Yui becomes Miaka's worst enemy after Nakago manipulates her into believing that Miaka doesn't care about her or her feelings for Tamahome.
A major force of tension in Gundam SEED is the fact that the two protagonists, Kira and Athrun, are best friends who have somehow wound up on opposite sides of a war. There is a great deal of introspection between the two of them about what this means to their friendship, how each other have changed, and why are they really fighting anyway? Eventually they both decide to Take a Third Option and wind up as part of the Three Ships Alliance, on the same side once again.
Muhyo and Yoichi with Enchu. The three were close friends in the academy, but Muhyo and Enchu were considered for the position of Executor; Enchu was desperate to get the position so that he could support his sick mother while Muhyo didn't want it. Enchu losing his mother and the position in quick succession was his Start of Darkness, as he became obsesssed with revenge against Muhyo.
Page and Tomas used to be friends before Tomas's evil nature became apparent.
Mariko and Aya in the anime of Oniisama e.... They got along in junior-high, but one day Aya threw Mariko under the bus in a school competition, and things never were the same from then on.
The "master and apprentice" variation of this went on in the Soul Society arc of Bleach between Yoruichi and Soi Fon, as Soi Fon worshiped the ground upon which Yoruichi tread before the latter fled for 100 years in an incident that made her an outlaw, leading to Soi Fon to swear that she would surpass Yoruichi and bring her in personally, although it is indicated that Soi Fon has forgiven Yoruichi. Canon is hazy about whether this is a Les Yay thing, however, which would change the dynamic significantly.
The thing is, it wasn't incident itself that Soi Fon was mad about: It was the fact that Yoruichi didn't take her with her when she left.
In Mai Hime, Shiho and Mai got along well early on in the series, with Shiho asking Mai to root for her attempts to win Yuuichi over. Midway through the show, however, Yuuichi tells Mai he has feelings for her with Shiho in earshot, and Shiho accuses Mai of lying to her and tries to kill her when her Hime powers awaken, even going so far as to kill Mai's younger brother out of spite.
In Axis Powers Hetalia we have America and England. Theirs is an interesting case, because even though America betrays England, it's done for political reasons, not because he hates him, and England is broken by it, but can't bring himself to kill him because he still loves him. Then, after hundreds of years they start to mend their bond. The shippers are not shy about what they think of all this.
In the webcomic form of the series, there was also Japan and China. China raised Japan as his little brother, only for Japan to break away by attacking China with a katana.
Depending on the mythos of that universe — and Depending on the Writer — Charles Xavier and Erik Lensherr (Professor X and Magneto, respectively) started out as best friends and bosom buddies, until they broke off because of differing opinions.
In at least one universe, the 'bosom buddies' business caused (or at least helped) both of their respective marriages to break up. This has not helped the people who protest the Ho Yay between the two at all.
The Juggernaut and Black Tom Cassidy from, thanks to Juggy's Heel Face Turn.
Colossus and Magik. Especially after he sacrificed everything (including his humanity and the love of Kitty Pryde) to save her from evil by taking on the power of Juggernaut only to find that, after her time in Belasco's Limbo, she had embraced evil, and only encouraged him to become the Juggernaut so he would become as damned as she was. When last he saw her, he promised, "If I ever see you again, I will kill you."
Spider-Man: Peter Parker and Harry Osborn. Their rivalry took place in the aftermath of the death of the original Green Goblin Harry's father and led to Harry going crazy and becoming the second Green Goblin. That a boobytrap set for Peter almost killed Harry's ex-girlfriend Mary Jane probably was seen by him as a positive side-effect.
In All Fall Down, the Pantheon tries to reason with Pronto this way. They don't quite succeed.
Steve Rogers and Tony Stark in Civil War, which was basically portrayed as a particularly volatile break-up/divorce. With Steve coming back from the dead, their relationship has since improved, not a little because fighting each other tore them both up pretty badly in the first place.
Watchmen Rorschach and Nite Owl II/Dan who were crime fighting buddies before the novel, now share a tensed relationship. Rorschach even somewhat disdains Dan's lifestyle, but as the story progresses they partly rebond and face the Big Bad together.
The entire point of the the sadly discontinued NanohaDark Fic 'Betrayers', which divides the cast into a bloody civil war. Spoken practically word for word by Fate.
In the Jackie Chan Adventures fic Queen Of All Oni, after Jade turns evil once again, her family and friends still care deeply about her, to the point that they are willing to FIGHT her so that they can return them to normal, even though her feelings towards them have been twisted by Daolon Wong's faulty spell, especially towards Tohru, since he became Uncle's apprentice instead of her.
In the Katawa Shoujo fanfic Reconciliation Hanako has this with Lilly and Hisao. The fic takes place after her bad ending, in which she snapped over being pitied by Hisao and angrily said she hated him and Lilly, then isolated herself from them out of guilt. This leads to an awkward moment when Hanako introduces her publicist and friend Sho to Lilly, who is unable to describe Hanako as anything more than someone she knew in high school, despite never having been angry with her for how she acted. By the end, the two have renewed their friendship and Lilly visits Hanako in Japan to catch up.
Obviously crops up in Inner Demons, as the entire plot is driven by Twilight Sparkle succumbing to her Enemy Within and becoming the new Big Bad her friends have to fight.
Also an element in Apple Bloom's subplot. Her friendship with the other Cutie Mark Crusaders (primarily Scootaloo) falls apart after an argument in which she makes it clear she wants to take her search for her Cutie Mark more seriously. Later on, Scootaloo and Sweetie Belle are talked intoFace Heel Turns, while Apple Bloom stays on the side of the protagonists. Notably, Sweetie Belle clearly still cares about their friendship and wants it back which is why she tries to talk Apple Bloom into switching sides as well.
In Naruto Veangance Revelaitons, this trope could be said to define Ronan and Taliana's relationship, along with Face Heel Revolving Door on Taliana's part. After several chapters of Taliana repeatedly switching sides, Ronan arbitrarily loses patience with her and gets a divorce from her. She makes several more attempts to kill Ronan for Madara, but he's still attracted to her despite her being his mother, and they have sex on a few occasions. Then the author broke up with the girlfriend who served as Taliana's inspiration, and Ronan kills her while claiming that he knew all along that she was evil.
In Weekend at Hisao's, Hisao ends up encountering his friends who, in the prologue to the game, stopped visiting after a few weeks, and hears that they have a get-together planned with his former crush Iwanako. It takes some coaxing from Shizune and Misha before Hisao decides to attend, but the meeting goes fairly well and his old friends end up getting invited to his wedding a few years later.
Rainbow Dash: Gilda! Tell me, why are you doing this? We used to be friends!
Gilda: That’s right, Dash! We used to be! But you chose those ponies instead of me!
In the Ponies of Olympus series, Rarity and her rival Ran Biao used to be friends when they were growing up together in Lao Wu's dojo, until Ran Biao grew jealous over Rarity's superior talent, and crossed the line by seducing and stealing Rarity's Love Interest Razorwing. Rarity never forgave her for that, and they've hated each other ever since.
In the Axis Powers Hetalia fanfiction Ignorance, My Bliss, the nations get energy boosts by drinking each other's blood, essentially making them all addicts. They discover that no one has fed off of America ever, meaning that his blood will give them a lot of strength, and don't hesitate at all to chase him down and drain him of his blood. Japan even tells America that though they're friends, his blood is just too tempting. And then, it turns out that even Canada turned on America, only pretending to save him so he'd have a chance to get him alone.
Star Wars: Taken to the extreme in Revenge of the Sith, the third film of the prequel trilogy during the battle between Anakin Skywalker and Obi-Wan Kenobi. See quote above.
13 Going on 30 deals with this. Jenna abandons Matt when she becomes more popular. They reconcile, though. Has something to do with Time Travel.
In Fire Over England, Sir Richard Ingolby, the English Privateer, unknowingly attacks the galleon of his former friend, Don Miguel de Casanz — when they encounter each other in battle, Ingloby tells his son, Michael, "We were friends once." "Enemies now!," shrieks Michael, and attacks.
The whole premise of X-Men: First Class is to show how Charles Xavier and Erik Lehnsherr became friends and then ended up on opposite sides with different ideals. It's a theme in the original trilogy as well, where they are Friendly Enemies and the beginning of X-Men: The Last Stand depicts younger - and closer - versions of them, when they were still working together.
Mark Zuckberg and Eduardo Saverin in The Social Network were once good friends but after Mark had Eduardo kicked out of the company, all bets were off.
In The Prince Of Egypt, Rameses and Moses were raised as brothers. They became enemies when Rameses was Pharaoh and Moses returned from hiding to free the Hebrews from bondage.
"You who I called brother,
how could you have come to hate me so?"
In Memoirs of a Geisha, the friendship between Sayuri and Pumpkin became strained once two rivals, Hatsumomo and Mameha, adopted each of them. And once Sayuri became more popular than Pumpkin, even having Mother formally adopted her, Pumpkin grew envious.
Peter Pettigew in Harry Potter. The other surviving Marauders view him with extreme disgust for siding with Voldemort.
In Warrior Cats, Fireheart and Onewhisker were friends ever since they were both young warriors. When Onewhisker becomes leader of WindClan, he is harrassed for being such a close friend to a cat of another Clan that he breaks off the friendship and views them more as enemies than anything.
The classic Dutch novel Oeroeg exploits this trope for drama in the relationship of the two main characters, a white Dutch boy of a rich family and a native Indonesian boy of a servant family. They grew up together, seperated during high school and when the main character finally returns to Indonesia (during the 1940s uprisings) he is told by his former friend to leave since he has no more business in the country.
Wesley Wyndam-Pryce from Angel. After Wesley translated a prophecy that predicted for Angel to kill his son, Wesley took the child to save his friend from his horrible fate. When he was about to leave town with the boy, he got his throat slit by The Dragon of Angel's nemesis and the baby was now taken from him and brought to a Hell dimension. His friends didn't take it that well. As a result to this, his friends never wanting to see him again, he takes a major in Badassery and is from then on very reserved concerning his former friends.
Gunn: What happened to you, man? Wesley: I had my throat cut and all my friends abandoned me.
Wesley steadily reconciles with them during the fourth and fifth seasons, however - despite the pain of what happened, Angel later states that overcoming this trope made their friendship afterward stronger than ever.
Also happens in the second season, when Angel fires Wesley, Gunn and Cordelia from Angel Investigations.
Buffy left her friends after she had to kill Angelus/Angel seconds after his soul was restored. She took off to Los Angeles, leaving her friends behind. Her return to Sunnydale is awkward - at best.
The entire point of Faith becoming The Dragon to the Mayor.
Cordelia also fits since 'Lovers Walk'. Though she doesn't fully repair her vitriolic friendship with the Scoobies completely, however, they manage a reconciliation by the end of the third season, and all interactions between them from then on are on generally good terms.
Rupert Giles and Ethan Rayne.
Michael from LOST is another good example. During Season 1, he seems to be a nice and well-liked member of the Losties. When his son Walt is kidnapped in the Season 1 finale, he knocks out Locke and Jack in order to get a head-start going after the Others. When he returns eventually, he decides to betray his friends and lure them into a trap to retrieve Walt.
General Hague in Babylon 5 is killed in battle after organizing a mutiny against President Clark's authoritarian regime. It's mentioned that the attacking ship was captained by a close friend of Hague's who had known him at the academy but nonetheless had remained loyal to Clark. The other officer is presumably killed when Hague's ship destroys it soon after. Major Ryan warns that this will be the case for most of their enemies in battle now.
Briefly lampshaded in the fifth season of Friends when Ross tries to work things out with Emily by agreeing to never see Rachel again. Phoebe and Rachel even discuss setting up a new 'group'. It didn't happen. Apparently.
The OC - Ryan Atwood at the end of Season 1 and the beginning of Season 2 - oh, and the beginning of Season 4. Leaving the Cohens after Marissa's death, becoming a cage-boxer, driven by revenge and grief along to Placebos 'Running Up That Hill'.
Also Seth claims to not care about Ryan and his family anymore, as they try to persuade him to come back to Newport at the beginning of Season 2. They manage to reconcile quickly.
Apollo and Starbuck from the Battlestar Galactica remake are a looping On-Off-Couple/Friends. When the fleet reaches New Caprica however and Apollo and Starbuck sleep together they seem to share a now very distant relationship. We only learn later on why.
Also happens to the Old Man und Col. Tigh. Twice actually. The first time when the exodus from New Caprica succeeds and Saul can't really deal with what happened, the second time when the XO reveals to Admiral Adama that he has been a Cylon all the time. They manage to reconcile eventually.
Doctor Who: The Doctor described The Master as having been his friend, once, before the latter went insane by staring into the Untempered Schism, a gap in the fabric of the Time Vortex itself. The first of the Tenth Doctor's two finale specials features reminiscences of their shared childhood. Apparently the Master was a rich boy: His father had estates. They played together on them.
The angel Balthazaar, a former friend of recurring character (and fellow angel) Castiel. At some point during the Apocalypse, Balthazaar went rogue and has started making deals with humans for their souls. When Castiel discovered this, and Balthazaar tried to get him to do the same, he stated how disgusted it made him, and declared the two of them enemies.
Gabriel and Lucifer. They used to be very close, as Lucifer claims that he taught his little brother everything he knows. Lucifer then kills Gabriel because Gabriel sided with humans.
Near the latter end of Season Six, Sam and Dean discover that Castiel is working with Crowley to gain control of Purgatory and all its souls. Castiel tries to justify his actions, but the brothers want nothing more to do with him (especially as of the season finale, where he jumped off the slippery slope). In an interesting inversion of the previous example, when Balthazaar discovers what Castiel is up to, he is horrified at what his friend is becoming, and starts spying on him for the Winchesters. This ends up costing him.
This really becomes a subversion because scenes showing him in a mental institution afterwards show that after the initial shock, his friends are actually very supportive, visiting him and trying to help him recover. They know that he's still their friend, he was just indoctrinated by what passes for an Evil Mastermind into doing his bidding because, despite his high intelligence, he is ridiculously naive and was taken advantage of. Everyone is more...baffled than feeling betrayed.
Also happens in Scrubs. Especially to Elliot and JD at the beginning of the fourth season. A milder case of the trope though. Their second attempt at a relationship ended badly and for a while they had a "superficial friendship", being generally friendly but not nearly as close as they once were. They became roommates, which brought them a lot closer.
Snow White and the Queen in Once Upon a Time, though due to the age difference it was more a niece and cool aunt relationship. As Snow White grew older and the Queen grew more bitter in her loveless marriage to the King, the two grew to hate each other. Also, the Queen blames Snow White for being tricked by the Queen's mother Cora into spilling the beans that the future queen was in love with Daniel the stable boy, leading Cora to kill Daniel to force her daughter to marry up.
Morgana discovers that Arthur is her half-brother in the third series of Merlin and spends the remainder of that series and all of the next trying to kill him so that she can consolidate her claim on the throne. When they finally come face-to-face at the end of series four, Arthur sadly asks her: "What happened to you, Morgana? I thought we were friends." When Merlin asked her a similar question in series 3, she replied that she hasn't forgiven him for poisoning her.
This trope is the only reason Merlin hasn't killed Morgana.
The Shield: With the end of Season 3 and the disbandment of the Strike Team, the relations between its former members are heavily tensed. Especially Shane, who seems to have crossed the line, thinks very differently about his former best friends:
Army: "Jesus Christ. Lem's one crazy-ass white boy. You really used to be tight?"
Shane: "Yeah. A lifetime ago."
Revolution: Captain Jeremy Baker with Miles, during the founding of the Monroe Republic.
Not to mention Miles and Monroe!
One of the plots between Jack Bauer and Tony Almeida in the seventh season of 24. Although the two originally started off on bad terms, Tony eventually became one of the very few people Jack personally trusted with his life. By season seven, a broken Tony has discovered the true identity to the conspiracy that led to the death of his family and his ties to the government, leading him to resort to terrorism in order to get the guy. Jack rightfully sees Tony as a traitor to his country betraying the memory of his loved ones, and Tony in turn sees Jack as nothing but a sellout and a hypocrite by continuing to work with the same government they've both lost faith in.
Rent - Roger, Collins, Maureen and Benny used to be tight when they were roommates, but when Benny moved out, bought their building and married Allison, a rift grew between them when it became clear he no longer thought the same way they did. Maureen hates Benny for "no longer believing in the same things." Mark and Roger hate him both for abandoning them after Roger got sick and for reneging on their deal. They all express themselves childishly. It's a stupid old world all around.
Inherit the Wind features a scene where Henry Drummond and Matthew Brady, opposing counsels in the trial over evolution, meet and muse over their past friendship. It's stated that their differing views on religion caused them to party ways.
Frank, Charlie and Mary had a tight friendship in Merrily We Roll Along until Frank made it big and the other two interpreted it as selling out.
Final Fantasy II: Firion, Maria and Guy with Leon. Even at the end, after his Heel Face Turn, he suggests that too much has happened for them to go back to the way they were before, but Firion holds out hope.
Final Fantasy VII: Cloud and Sephiroth. Or at least that's how Cloud initially frames how they know one another. In reality, Cloud and Sephiroth were never friends, but Zack and Sephiroth were and did act out this trope.
And in Crisis Core Zack and Angeal in addition to Sephiroth and Genesis.
Metal Gear Solid 4: Big Boss and Major Zero's We Used To Be Friends is partially the cause of the Metal Gear Saga. Over time, they went from close friends to the most bitter of enemies. Why? because they were both so arrogant in their opposing (wrong) beliefs about what their idol, The Boss, wanted for the world that they each went to extremes to fulfill their personal interpretations, dividing The Patriots and causing Zero to create Solid and Liquid. Whilst at the end Big Boss no longer feels hate for his old comrade, it is far too late for them to repair their friendship as Zero's advanced age had left him a brain dead shell.
Shepard and the Virmire Survivor in Mass Effect 2. Joining up with terrorists (no matter how noble your intentions) will do that.
Worse at the start of Mass Effect 3, although they're prepared to hear you out. Depending on whether you take the time to clear the air, they can end up rejoining you, going to work with Hackett or, uh, dying at your hands. That last one's not too hard to avoid, though.
In Ace Attorney, Phoenix Wright and Miles Edgeworth were part of a Power Trio with Larry Butz during the fourth grade after Miles and Larry were the only ones to stand up for Phoenix when he was accused of stealing Miles' lunch money. It was Larry. When something smells, it's probably the Butz. Then Edgeworth switched schools after his father was murdered and switched career goals from defense attorney to prosecutor. Phoenix was inspired by Edgeworth and Butz to become a defense attorney himself, which led to a legendary courtroom rivalry between Wright and Edgeworth. They do become somewhat friends by the final case of the first game when Edgeworth actively helps Wright get the true killer of a case to confess.
This also happens with Lana Skye and the rest of the SL-9 investigation team. Because she was blackmailed by Gant into fabricating evidence, and she became cold and distant as a result, Angel Starr and Jake Marshall became quite bitter toward her.
In Dead or Alive Dimensions, a few scenes suggest this of Kasumi and Ayane, as flashbacks show the two playing together, and their mother, Ayame, tells Ayane that they used to get along quite well, and hopes they will again.
In Tales Of The Abyss, Jade and Saphir (who now goes by the name Dist the Rose) are this trope, although Jade denies it and Dist claims to regret it ever having been true. Their past association was broken when Jade chose to abandon fomicry due to personal and ethical concerns, while Saphir refused to stop trying to revive Professor Nebilim, considering Jade a traitor for giving up too soon. The party speculates they like each other better than either wants to let on, and it's usually Played for Laughs.
In Guild Wars 2 this is the current state of the (surviving) members of Destiny's Edge, the Five-Man Band of Living Legend s that takes up a lot of the backstory. The human member of the group abandoned them during a crucial battle to save his queen, which ended up getting one of their members killed. He is saddled with the brunt of the blame for what happened, but it also brings out and creates a lot of resentment for some members towards others, and they drift apart. Particularly tragic since the group was quite important in the whole overcoming racial barriers department. A large part of your characters personal story is based around getting the band to overcome the bad blood and get back to the whole saving the world from dragons business. (It is an MMO after all, obviously that task couldn't lay on your shoulders alone.)
Katawa Shoujo has Lilly and Shizune, who are also cousins. Lilly joined the Student Council when most of the older members had left, but got into many arguments due to their personalities clashing (Lilly thought Shizune was too harsh and tyrannical, while Shizune thought Lilly was too unreliable). In Lilly's route, the two reconcile shortly before Lilly is to leave Yamaku. In Shizune's route Shizune admits that she was in the wrong.
In Knights of the Old Republic Revan and Malak used to best friends, till they were corrupted by the Sith Emperor. Then the dark side of the force did the rest.
Retroactive example in Homestuck: Vriska actually used to be pretty close to Aradia and, in particular, Terezi, but after she started going too far and becoming Chaotic just for the sake of it, both turned their backs on her, their relationship irretrievably broken. Even later when Terezi remembered their friendship and wished they could return to it she couldn't pretend that Vriska wasn't evil or that she wouldn't work against her if it was necessary.
In Evil Plan'' Stanley Kinesis, (back in college) and William sever ties after a fight about the telekinetic chip. Possibly subverted, as it was recently revealed that they're still on speaking terms. Why this is has not yet been explained.
In Sonic The Comic Sonic and Amy were friends but in Sonic The Comic Online after the Kane arc Sonic and Amy have become enemies (although it is implied Amy still trusts Sonic on some level, it's just impossible to be sure about him given the circumstances).
In The Gamers Alliance, Vaetris and Jahi were once prostitutes in the Demon Realm and close friends, but things changed when they were abused too much and Vaetris's ambition to change their living conditions turned her to the path of conquest, bloodshed and manipulation whereas Jahi chose to nurture her young instead, which set the two on different paths. Ronove and Nesa became friends while the former was amnesiac but once Ronove regained his memories of being a demon lord, he took back his position in the demon hierarchy and ultimately betrayed his friends, Nesa included. However, both cases are more complex and grey than that and have shades of Friendly Enemy because neither Vaetris nor Ronove act in a truly villainous way and they do seem to care for their former friends somewhat whenever they're dealing with them although in both cases manipulation is also involved.
In Gargoyles this occurs between Macbeth and Demona, as seen in "City of Stone": best of friends when they first fought together against Duncan and won Macbeth his kingdom, things go south when Demona, mistakenly believing that Macbeth was about to betray her and her clan, betrays him first, causing him to lose his kingdom, family, and identity.
Occurs in the second Teenage Mutant Ninja Turtles 'toon between Leonardo and the Shredder's adopted daughter Karai, who in previous seasons had developed a rapport of sorts with him, despite their Feuding Families. When the turtles successfully stop the Shredder's plan to leave Earth to conquer the Utrom homeworld, and end up getting him imprisoned and exiled, Karai takes it as a betrayal, and vows to avenge her father. Later, after Karai destroys the turtles' lair and believes she's killed the other turtles and Splinter, she heads into April O'Neil's shop and asks her to give Leonardo a message. When April asks Karai (whom she's never met before) how she knows the turtle, she responds with the trope name.
Plankton and Mr. Krabs of Spongebob Squarepants. They used to be close friends but are now mortal enemies because of the Krabby Patty formula.
Avatar The Last Airbender reveals that Aang's predecessor, Avatar Roku, used to be best friends (with a lot of Ho Yay,) with Firelord Sozin, the man who started the Fire Nation's war for world domination, and committed genocide against the airbenders.
Rainbow Dash and Gilda in "Griffon the Brush Off". They used to be childhood friends, but their friendship is broken off after Dash finds out that the latter has been bullying her other friends behind her back.
Rarity and Sweetie Belle in "Sisterhooves Social", thanks to the latter's poorly-executed attempts to help out with Rarity's house chores. This results in Rarity becoming more and more frustrated with her younger sister, and eventually an argument breaks out where Sweetie Belle calls her big sister out for being ungrateful. The fight ends with the two deciding that neither of them wants the other as a sister anymore.
Twilight Sparkle: I was just trying to- Shining Armor: She's been completely stressed out because it's really important to her that her big day be perfect! Something that obviously wasn't important to you. Now if you'll excuse me, I have to go and comfort my bride. Oh, and you can forget about being my best mare. In fact, if I were you, I wouldn't show up to the wedding at all. Applejack: C'mon, y'all. Let's go check on the princess. (Everypony coldly walks past Twilight. Celestia is the last to leave.) Princess Celestia:I would have a lot to think about!
Later a subversion. When Twilight is proven to be right all along, Applejack apologizes for ditching Twilight on everyone's behalf. Twilight holds no grudge.
Often found in political discussions and developments. Former associated politicans changed their views or motivations.
This troper being from Germany has to think of Gerhard Schröder (who would become Chancellor of Germany years later) and his fellow social-democratic party (SPD) buddy Oskar Lafontaine who had an intense fight spanning years due to differing opinions.
Pete Doherty and Carl Barât of The Libertines seem to have estranged over the years.
Before Genghis Khan was Genghis Khan, he was Temüjin, and he was best friends since child hood with boy named Jamukha. Eventually, Jamukha was declared a Khan himself. Temüjin refused to accept this, and the two who had been as brothers went to war with one another, with Temüjin being victorious due to his more progressive methods and tactics, along with Jamukha's forces betraying him. After rewarding the traitors, Temüjin offered the defeated Jamukha a place by his side so that they could resume their friendship, but Jamukha declined with the following speech;
Jamukha: What use is there in my becoming a companion to you? On the contrary, sworn brother, in the black night I would haunt your dreams, in the bright day I would trouble your heart. I would be the louse in your collar, I would become the splinter in your door-panel...as there was room for only one sun in the sky, there was room only for one Mongol lord."
Then he asked to be killed without having his blood spilled. Temujin's soldiers broke his back.
Paris Hilton and Nicole Richie. Big time.
Sigmund Freud and Carl Jung. After a few years as colleagues, Jung tried to convince Freud that focusing purely on the sexual roots of human motivation was too limiting. He believed that the subconscious is not so much the repository of repression as it is the wellspring of creativity. He also promoted the idea of the Archetypal Character, and that many actions are based on patterns that tend to repeat themselves. (Perhaps this makes him the ultimate Trope Codifier of ... well, of tropes.) Freud disagreed with what is now called Jungian psychology because, naturally, he believed that All Psychology Is Freudian.
Friedrich Nietzsche and Richard Wagner. Nietzsche even wrote two books explaining his perspective: The Case of Wagner to mark his break, and Nietzsche contra Wagner to explain how his thoughts on music had been consistent, but his earlier application of them to Wagner had been incorrect.
Shaquille O'Neal and Kobe Bryant
Shaq and Kobe were never friends. At best they tolerated each other for the sake of winning championships.
Harry Houdini and Sir Arthur Conan Doyle were very good friends who bonded over a shared interest of the Occult, Spiritualism to be exact. The two would go around looking for people who claimed to be psychics or mediums or witches, etc, but where Houdini was an Open but heavy Skeptic, who believed everything to be fake until proven otherwise, Doyle tended to believe everything at face value (ironic). The two eventually went different ways after Houdini declared Doyle's wife, who claimed to be a Ghost writer, a fake.
David Letterman and Jay Leno. Leno helped inspire Letterman to try comedy, then Leno became one of Letterman's early supporters. Then Leno was Letterman's most frequent guest during the early years of Late Night, which boosted Leno's profile and led to him becoming Johnny Carson's fill-in host. Then Carson retired and NBC picked Leno to replace Carson, a job that Letterman considered himself the heir-apparent for.
Lil' Kim and Nicki Minaj used to be on good terms, believe it or not. Nicki Minaj used to give a shout out to her in interveiws as a inspiration, and there are pictures of them smiling and putting their arms around each other at a concert. Minaj also said she was a die hard fan of her as a kid. Then, for reasons ambiguous, they became enemies and wrote various songs targeted at one another defaming the other, and calling each other very profane words. They still put references to how much the other one sucks in their mix tapes, and are sure to bring the other one up in the interviews even when not asked.
This probably happened to anyone who used to be friends with someone but can no longer be as such due to betrayal, betrayals, general growing apart, personal growth or lack there of or a similar event or EVENTS.
Another notable historical example was William Marshal, known for actually keeping to his word in an era of political expediency. He followed Henry II's eldest son, then followed Henry II, even when Richard (the Lionheart) rebelled against his father. The trope comes in due to William tutoring Richard and even being good friends, yet they came face to face during the retreat of King Henry's army. William Marshal, owing to being perhaps the most skillful warrior in Christendom at the time, was able to kill Richard's horse and let the man live.
John F. Kennedy and Richard Nixon were friendly while serving in Congress together. Kennedy reportedly donated money to Nixon's 1950 Senate campaign, and the two retained offices across the hall from each other after Nixon became Vice President. The presidential campaign in 1960 ended their friendship; Nixon felt especially angry that when he was hospitalized, Kennedy neither visited him nor suspended his campaign. For Nixon, this mutated into near-pathological hatred of not only JFK but his brothers as well.
Nixon was actually most upset about how JFK probably stole the 1960 election.
Vladimir Lenin and Josef Stalin were friends thanks to their beliefs, but as Stalin became more violent, Lenin wrote to TROTSKY to ask him to unite to try and get rid of Stalin, who Lenin thought was getting out of control.