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.
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.
Dual Destinies gives a few examples, mainly in the third trial. Athena, upon being reunited with her childhood friend, Juniper, is surprised to see Junie acting coolly towards her. As the case goes on, it turns out that Juniper and two other students, Hugh and Robin, had also begun to split apart after being inseparable. It turns out that this is all because a number of secrets, including Juniper acting as a spy to report wrongdoing in the school, Hugh learning his parents were bribing the teachers to improve his grades, and Robin actually being a woman, put so much stress on them that they became distant towards one another. They all reconcile by the trial's conclusion, happily.
In the UPEO-General Resource path, Abyssal Dision is revealed to be the leader of the terrorist organization Ouroboros. Eventually he comes to blows with Keith Bryan, his former wingman, who says to Nemo that hes not the Dision he knows.
In the UPEO-General Resource-Ouroboros path, Nemo will fight both Dision and Keith, as Keith sacrifices himself to destroy the Night Raven, causing Dision to lose his sanity, forcing Nemo to shoot him down.
In the UPEO path, when Fiona escapes with Gabriel Clarkson to defect to Neucom, either Nemo or Rena will shoot down the transport plane shes flying.
In the UPEO-Neucom path, Rena becomes this for Nemo after defending Fionas transport plane and defecting to Neucom.
In Ace Combat 5: The Unsung War, in the final act of the game Pops is revealed to be a former fighter pilot of Belka, and he and the squadron leader of the 8492nd squadron used to be comrades in arms during the Belkan war.
The final boss of Ace Combat Zero: The Belkan War, is Solo Wing Pixy. Ciphers former wingman from the beginning of the game, who became disillusioned with the war after the bombing of Hoffnug.
Rios: I've changed my mind. I don't want to see that fucker's face again. Alpha: You managed to even make Rios hate you, you evil sack of shit.
In A Way Out, this happens to the two protagonists after Vincent reveals he's a cop and he will arrest Leo. Leo naturally freaks out about this revelation and the rest of the game involves the two having a duel to the death. Unfortunately, one of the protagonists has to die and there's no opportunity to mend their broken friendship.
BattleTech has Kamea Arano and her cousin Victoria Santiago. The latter betrays the former at the behest of her father, usurping her throne because she believes Utopia Justifies the Means.
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.
Dead Rising: Frank eventually meets Kent, a fellow photographer whom he gets along quite nicely with. Until Ken seems to crave a little too much for sensation.
In the finale of DmC: Devil May Cry, Vergil, Dante, and Kat stand atop the ruins of the collision between Limbo and the real world, having just ousted Mundus and his demonic grip on society. It is at this time that Vergil reveals all you have been fighting for is so he can take the throne from Mundus and begin his own new world order to subjugate humans. Dante and Kat are shocked and outraged that they just killed an emperor only to crown a new one and Dante steps up to make sure that doesn't happen. Vergil is genuinely surprised that Dante wants no part of his vision and the two come to blows.
Varric and Anders by the time of Dragon Age: Inquisition. After Anders' atrocity at the end of Dragon Age II, if Anders survived the final battle of that game, if asked, Varric bluntly says that he has no idea where Anders has gone, and frankly, he doesn't care either way.
Ensemble Stars!: The War split up a number of different characters onto opposite sides:
The most dramatic change was probably that of Keito and Rei - Crossroads indicates that they used to be very close until Keito decided to choose his childhood friend Eichi over him, even if it meant sabotaging him. In the present day, they are generally cold to each other.
Kuro and Shu also used to be childhood friends; the latter now bears a big grudge against the former for his actions against the other Oddballs as part of Akatsuki.
Natsume and Tsumugi also used to be childhood friends. Like Shu, Natsume is still very bitter about Tsumugi's role in the war (in fact, Natsume was chosen as an Oddball by Tsumugi's suggestion), but deep (very deep) down he does still want to be friends with him.
Leo and Eichi used to be extremely close, with the former nicknaming the latter 'angel', until Eichi's attempts to push him resulted in a mental breakdown far beyond what anyone expected and Leo disappeared for several months. Neither appears to actually hate the other but interactions are awkward at best now.
Averted with Eichi and Wataru - though they were friends and Eichi did betray him, Wataru still respected him highly and quickly forgave him, ultimately convincing him not only to keep fine as a unit but also to let him join.
The Elder Scrolls V: Skyrim: Ulfric Stormcloak, Galmar Stone-Fist, and Legate Rikke were once friends who fought together in the war against the Aldmeri Dominion. In Skyrim's Civil War, however, Ulfric and Galmar lead the Stormcloak rebellion and Rikke works with the Imperial General Tullius to put said rebellion down. There is no happy ending for them. If the Stormcloaks win, Ulfric and Galmar all but plead Rikke to surrender so they won't have to kill her, but she refuses. If the Imperials win, Rikke still hopes to the bitter end that Ulfric will see reason and surrender, but is forced to fight him and Galmar to the death when he also refuses.
Paarthurnax, in the ending of the main quest, grieves for Alduin with words very much like this.
Petra and Jinjur from Emerald City Confidential have this dynamic when the two part ways after their army days. Jinjur is bitter because Petra abandoned her post and duties during the Phanfasm War to find her missing brother. Petra is bitter because the Ozian royal guard never helped her search for him.
Fallout: New Vegas: This is the general sentiment between Boone and Manny, two former First Recon Snipers, who moved to Novac to start a new life together, after leaving the army, but had their friendship tested, due to disdain between Manny and Carla, Boone's wife, possibly due to Manny'sown feelings for Boone. This came to a head when Carla went missing under mysterious circumstances and Boone realized Manny was happy his wife was gone. By the time the player comes across them in Novac, they are understandably not on speaking terms.
Final Fantasy II: Firion, Maria and Guy with Leon. Even at the end, after his HeelFace Turn, he suggests that too much has happened for them to go back to the way they were before, but Firion holds out hope.
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.
In the prequel Crisis Core, there's Zack and Angeal in addition to Sephiroth and Genesis.
Final Fantasy X-2: Paine, Nooj, Baralai, and Gippal. Nooj getting possessed two years ago and shooting at the rest of them pretty much destroyed their friendship, and theres still resentment over that when your party eavesdrops on the latter three late in the game.
In Fire Emblem: Three Houses, once the Flame Emperor is unmasked as Edelgard, depending on the route, several friendships can be permanently destroyed.
Recruiting characters to and from the Black Eagles can cause several groups of friends to end up on opposite sides of the ensuing war (Caspar/Linhardt, Dimitri/Sylvain/Felix/Ingrid, Mercedes/Annette, Ignatz/Raphael, Shamir/Catherine).
Edelgard was once Dimitri's beloved stepsister, but after she is revealed to be the Flame Emperor he swears revenge on her due to her perceived role in the Tragedy of Duscur and all four story routes result in one of them dying at the other's hand. Dimitri does express explicit interest in reconciling with her late in Azure Moon, but by that point there's no going back for her. Interestingly, this particular example is actually subverted on Edelgard's part, as she doesn't actually remember most of her relationship with Dimitri thanks to her Dark and Troubled Past. She does remember completely towards the end of Azure Moon, but acknowledges at that point that the Edelgard he knew back then is effectively dead and says their differences in beliefs means they're still enemies.
Edelgard's (and to a related extent her loyal retainer Hubert's) friendships with Byleth and the Black Eagles are ruined if Byleth is a Black Eagle and chooses to side with the Church of Seiros. Their reunion in Silver Snow has her solemnly declare that despite their past relationship, they must now be enemies for the sake of her ambitions.
If Byleth is a Black Eagle and sides with Edelgard, any friendships they or the Black Eagles made with the staunchly Church-aligned characters (Flayn, Seteth, Cyril, Catherine, Rhea) are destroyed. While you can sort-of patch things up with Flayn and Seteth and spare Cyril and Catherine's lives, Rhea is the route's Big Bad and must be killed.
This is driven home in Part II's return to Gronder Field, the site of a mock battle between the houses five years prior. While Dimitri has gone Ax-Crazy and threatens to murder anyone who stands in his way, Claude and Edelgard lament that they are now forced to fight and kill their former classmates.
Grand Theft Auto: Quite popular in the series. GTA III featured Claude and Catalina, Vice City features Tommy Vercetti and Lance Vance, San Andreas has CJ and Sweet Johnson with Big Smoke and Ryder. Grand Theft Auto V takes it up a notch by letting you turn against your friends. At the end of the game, Franklin's life is on the line and he is forced to kill either Michael or Trevor to save his own skin. If you choose to kill Trevor, Franklin tries to justify his actions by saying how dangerous and psychotic Trevor was to everyone, which causes Trevor to blow up and say how he and Franklin used to be good friends to each other. Likewise, choosing to kill Michael will have him angrily blow up on Franklin, stating how he treated him like his own son and showed him how to make a big score. Franklin weakly justifies his actions towards Michael by saying he had it coming the whole time. You can also defy this trope and team up to kill all of the villains instead, which results in the three amicably parting ways.
In Guild Wars 2 this is the current state of the (surviving) members of Destiny's Edge, the Five-Man Band of Living Legends 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.)
In Ikemen Sengoku, Shingen used to be best friends with the monk Kennyo but the latter broke off their friendship after being Driven to Villainy by Nobunaga burning down his temple and becoming even more ruthless than him in the process. Shingen still harbors hope that they can become friends again someday, but the other man considers himself too far gone for that and would rather not have Shingen see him like this if possible.
inFAMOUS and its sequel have this with Cole and Zeke. Both were best buds for the longest time, but then the Ray Sphere explosion happened, and things went downhill for everyone. Zeke became envious of Cole's powers, and at one point betrayed him to try and get his own. Cole ends the first game feeling more alone than ever, despite having various allies helping him, including Zeke. By the second game, they're doing better, but Cole is not completely trustworthy of Zeke. By the end, they can patch things up, depending on your actions.
Zeke: I don't call you 'Brother' cuz I like the way it sounds...
However in the Bad Ending of the second game, the final battle is a woefully outmatched Zeke versus a godlike Evil!Cole. He knows he stands no chance, but he still has to try to stop Cole's reign of terror.
When first meeting Cole, Sebastian Wolfe states that he used to be friends with Kessler. Their friendship was apparently genuine as it's implied that Wolfe somehow knows that Kessler was a future version of Cole.
Injustice: Gods Among Us sees an alternate Superman fall to the dark side after the Joker drugged him and tricked him into killing Lois Lane—which in turn destroyed Metropolis as the Joker hooked up a nuke to her heart and Batman forming a resistance after most of the League decided to join Superman in taking over the world. However, Bruce does regret how things went, and the sequel shows the feeling is mutual with the duo somberly reminiscing about old times before they fight for the final time:
Superman: Fighting on the same side... felt like old times. But I guess we both knew it would end this way. Batman: Do you remember that night? When you told me Lois was pregnant? Superman: You knew. Even before I said anything. Batman: That was a good memory. Superman: From another lifetime. Batman: I miss the people we were then. Superman: Me too.
Iron Marines has one in the final mission of Borealis. If your chosen Hero Unit is Fate, a defected Raad, Raad commander and Arc Villain Nexus will recognize them as a former Raad ally who left for personal reasons despite all they have gone through and feel very betrayed over it.
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.
Misha is deathly afraid of this happening to her, Shizune, and Hisao. They're graduating high school soon and Misha is scared they'll go there separate ways and Shizune will eventually forget her. In the beta version this is a reason why Misha is Driven to Suicide.
Sora and Riku. Riku becomes a major antagonist after Maleficent manipulates him into believing that Sora doesn't care about him or his feelings for Kairi. He soon snaps out of it, and his friendship with Sora is restored. Of course, Sora never thought of Riku as an enemy, and after he saves Kairi, is determined to rescue him.
Sadly played straight with Aqua to her friends Terra and Ventus. Aqua and Terra take the Mark of Mastery exam together, but only Aqua passes due to Terra losing control of the darkness within his heart. Eraqus gives the two of them a mission, which, for Terra, is a second chance to prove himself worthy, but also secretly tells Aqua to bring Terra back if he starts to succumb to darkness, and tasks her with finding Ventus when he goes after Terra. While Aqua tries to believe in Terra and even argues in his favor, she also becomes concerned about his actions, and Terra and Ventus don't take it well when they hear about Aqua's mission. This dispute is never fully resolved for the rest of the game, but the friends continue to care for each other, to the point of being willing to sacrifice just about anything for each other.
Also from Birth By Sleep, there's Master Xehanort towards Master Eraqus. The two were fellow Keyblade apprentices, but Xehanort injured Eraqus during an argument. Later on, there appeared to be some reconciliation when Eraqus willingly agreed to look after Ventus for Xehanort, not knowing that Ventus had ended up that way as a result of Xehanort forcibly extracting the darkness from Ventus. Eventually, Eraqus realizes that Xehanort is plotting something, and planning to use Ventus to make the X-blade, so Eraqus tries to kill Ventus, resulting in Terra fighting and defeating Eraqus and Xehanort finishing Eraqus off.
Axel and Saïx, back when they were Lea and Isa. As they grew up and became Nobodies, Saïx became crueler due to Xehanort's Grand Theft Me and Axel befriended Roxas and Xion instead. This all comes to a head in 3D, where Isa has been completely taken by Xehanort and attacks Lea without hesitation at the climax of the story.
In Knights of the Old Republic, Revan and Malak used to be friends, till they were corrupted by the Sith Emperor. Then the dark side of the force did the rest.
In Lost Horizon, Fenton is not keen on having to go see Yen Wuang, who was a friend until Fenton was publicly accused of causing several deaths. Fenton claims that Wuang left as soon as possible and gave Fenton no chance to explain himself, indicating that Wuang wasn't even willing to hear him out. Wuang's niece Kim disagrees, saying that Fenton could easily have found them to explain if he'd wanted to and that his refusal to try left them to assume that the press reports must be accurate. Since Wuang is now dead, it's too late for them to reconcile (although Fenton and Kim do).
Shepard and the Virmire Survivor in Mass Effect 2. Joining up with terrorists (no matter how noble your intentions) after apparently faking your death will do that. Especially when the leader of the terrorist group in question does not want your old friends anywhere near you.
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 dying at your hands. That last one's not too hard to avoid, though.
James Vega: You know the Commander? Virmire Survivor: I used to.
The backstory to Dr. Light and Dr. Wily in Mega Man 11 shows the two used to be friends during their days at robotics university. This came to an end when Wily proposed the Double Gear system, which would overclock robots' systems to increase their strength and speed far beyond their normal limits, enabling them to become great heroes. Light disagreed, saying that such power would put a dire strain on a robot's body, and could easily be misused with disastrous consequences. The research committee agreed with Light, and canceled Wily's project in favor of funding Light's own research into self-aware AI. Wily never forgave Light for crushing his dreams, and from that day on he spent the rest of his life trying to outdo his former friend to prove him wrong. As Light explains these events to Mega Man, the doctor wonders if things would have been different if he had shown Wily how they could have worked together instead of telling him he was wrong. In the end, Light tries to appeal to Wily this way, showing how great a hero Mega Man has become using both Light and Wily's respective technologies, and how they could combine their efforts like old times once more. Wily makes it very clear that bridge was burned for good and that by now all he cares about is proving himself right and Light wrong no matter the cost.
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.
Minecraft: Story Mode: Something happened between Ellegaard and Magnus when the Order split up, and they're not happy to see each other when the party reunites in the second episode.
A villainous example lies with Kabal and Kano in Mortal Kombat 9. After saving Kabal's life after being nearly burned to death, Kano expects his fellow Black Dragon partner in crime to come back to his gang. Having become The Atoner with joining the police force, and clearly horrified in what Kano turned him into, Kabal doesn't exactly reciprocate Kano's offer. But when he tries to confront Shao Kahn, Kabal opts to knock Kano out rather than kill him.
In his arcade ending from Mortal Kombat X, Bo'Rai Cho ends his friendship with Raiden after learning about the corrupted god's unprovoked attack on Outworld, training Outworld warriors to oppose him.
Moshi Monsters: Elder Furi (a wise old monster) and Dr Strangeglove (the main villain) used to be friends until they graduated university, at that point Strangeglove turned evil.
Nuclear Throne has this between Rogue and Li'l Hunter. They used to spend a lot of time together in the IDPD, but at some point Li'l Hunter was sent on a mission alone, failed it, and was summarily abandoned by the IDPD. Some Sanity Slippage later and he now no longer remembers Rogue, but she still remembers him.
Barham and Orlick were once friends/research partners until Orlick's obsession in a research regarding a particular object caused a disagreement between them that resulted in their falling out. During Therion's story that involves taking the very object Orlick has been obsessed with researching, Barham, despite his fallout with Orlick, still harbors hope that he and Orlick will bury the hatchet and helps out Therion because of said hope. After Therion's story arc ends, the player can find Barham and Orlick together, having made up and become friends/research partners once again.
Knoth sees Val this way in Outlast II. Val was once one of the Sinister Minister's cultist followers until she broke free and formed The Rival heretics. Although she swears them enemies and sent him an insulting letter, Knoth still has a willingness to forgive her for this.
In Overwatch, Gabriel Reyes (the man who became Reaper) used to be very good friends with Jack Morrison (Soldier: 76). However, Morrison's status as the leader of Overwatch made Reyes extremely jealous, since he felt he was more qualified. Reyes started secretly working against the organization, culminating in him trying to stage a coup to overthrow Morrison. The resulting destruction from their fight caused an explosion, which turned Reyes into a vengeful, undead being and cemented his FaceHeel Turn.
Persona 3: Happened to Akihiko/Mitsuru and Shinjiro, though without the FaceHeel Turn. The three were the founding members of SEES, but after an incident in which Shinjiro's Persona went out of control, killing Ken's mother, Shinjiro distanced himself from the group, resulting in many bitter quarrels with Akihiko. Later in the game, Shinjiro rejoins the team, but things remain tense between him and Akihiko until Shinjiro dies protecting Ken.
Persona 4 Golden: Under the right conditions, namely getting the Jester Social Link to Rank 6 before November 5 and then confronting the killer alone, you can invoke this on the killer when their identity is revealed. The killer calls you out on your naivete and threatens to shoot you.
Also from the fourth game, there's Chie's friendship with Takeshi, whom you meet on Chie's Social Link. Chie and Takeshi have known each other a long time, but Chie is somewhat uncomfortable with Takeshi's infatuation with Chie's best friend, Yukiko, and it's clear that Chie and Takeshi are more distant than they used to be. Later on in the link, though, during an argument over Takeshi's refusal to admit that he ran away from a gang of bullies, Takeshi notices how Yukiko has changed and become more openly cheerful, laughing at the strangest things, and accuses Chie of causing this. Chie angrily points out that this is Yukiko's real personality, and the two part on bad terms, never to be seen together again.
Red and Blue from Pokémon Red and Blue were friends however as they grew they became rivals due to their similarities (same height, same age, etc). Future games depict them as having mended their friendship.
Similar to this is Archie and Maxie from Pokémon Ruby and Sapphire's friendship before they split off into Team Aqua and Team Magma.
In Portal 2, Wheatley attempts to exploit this to make Chell stop running away from his Death Trap. Being Wheatley, it's not very subtle.
"Aw. Just thinking back to the old days when we were friends. Good old friends. Not enemies. And I'd say something like 'come back', and you'd be like 'no problem!' And you'd come back. What happened to those days?"
Basically the plot of Red Dead Redemption, where outlaw John Marston is forced by the government to track down and arrest/execute his old gang companions. He does try to downplay how close he was to many of them, especially since they've gotten progressively immoral since his departure, but it is clear he takes no joy in his job. To twist the knife further, the government agents send an army to take him out after he finishes.
Resident Evil 4: Leon and Krauser, where they aren't happy to see each other after several years since their last meeting. Their soured relationship is further explored in Resident Evil: The Darkside Chronicles where Krauser sustained a nasty injury on a mission with Leon and was forced to retire from his unit while Leon, who was a fresh government agent, got awarded for his actions in the mission. Krauser's retirement left him without a job and he became resentful towards Leon. When Wesker offered him a job as a mercenary, Krauser took the chance knowing that he would run into Leon eventually and finally settle the score. Leon believed him to be dead for years and remained oblivious to this bad blood until Krauser finally made his reappearance.
In Shovel Knight the titular character and Polar Knight used to be friends. Both encourage the other to change allegiances before fighting, and even after his second defeat the Polar Knight still believes Shovel Knight should join the Order.
There's also Black Knight. Prior to the events of the game, they were in the same team as Shield Knight, and Black Knight's attempts to hinder Shovel Knight were meant to prevent him from killing Shield Knight (who has become possessed by The Enchantress).
One Sunken Scroll in Splatoon reveals that the Inklings and the Octarians used to have friendly relations before the Great Turf War broke out over a lack of livable territory. Specifically, it shows a younger Cap'n Cuttlefish hanging out with a humanoid Octarian who appears to be DJ Octavio.
Early on in The Stretchers, the Operator mentions that Captain Brains used to be a medic who worked with the ambulance driver protagonists before turning to his current villainous ways.
Strider Hiryu and Strider Hien has this relationship in Strider 2: they were close friends who worked several missions together in the Striders, until Hien got seduced by the dark side and turned to the Grandmaster's side, assisting his forces into wiping out the Striders entirely. Hien still considers Hiryu a friend, however, and after their first fight he tries his best to convince Hiryu that his mission is ill-fated from the beginning and to just forget it, but Hiryu simply turns him down and advises him to fight at his best next time. By their second (and last) encounter, Hien wastes no words and fights him all-out.
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.
The Wizard and the King in Teslagrad used to be staunch friends and allies, but then the King began to abuse the powers the wizard had given him...
Undyne in Undertale will strongly feel this way if you befriended her and then kill someone afterwards. The only reason she accepted your friendship in the first place is that you spared her and then went out of your way to get to know her, even though she tried to kill you because you were a human. Undyne's king, Asgore, needing human souls to free all monsters was also part of the reason why she was hostile towards you. Killing anyone after befriending Undyne has her stop being your friend and, according to Papyrus, would utterly destroy you if you ever showed your face in her neighborhood ever again.
In Deltarune after beating the Bonus Boss, you have the option to tell Seam the shopkeeper about it. Seam will wistfully say that they and Jevil were friends who enjoyed each others' company and played games together, but one day Jevil was driven mad after being told that the world he resides in is a game, and Seam had to lock him away for the good of the kingdom. Dialogue between the two shows that both parties take this in stride.
In World of Warcraft, 25,000 years ago, Kil'Jaeden and Velen were best friends, almost brothers. They, along with Archimonde, ruled the world of Argus. Then came Sargeras and offered the three (and, by extension, the entire eredar race) a place of honor in his Burning Legion. Archimonde accepted out of hunger for power, Kil'Jaeden agreed out of thirst for knowledge, but Velen saw the Burning Legion for the evil it was. With the help of creatures of light called the naaru, Velen and a number of his followers fled Argus in a ship. Considering Velen to be a traitor, Kil'Jaeden vowed to hunt down and exterminate all the escapees, now calling themselves "draenei". As far as he's concerned, Velen betrayed him personally.
Kazuma Kiryu and his sworn brother Akira Nishkiyama from the Yakuza series; as shown in Yakuza 0, Kiryu and Nishiki were Bash Brothers when they were first starting out, the two of them fighting together against the Dojima family despite almost insurmountable odds. Then Nishiki killed Sohei Dojima in the opening scenes of Yakuza, with Kiryu taking the fall so Nishiki could continue to support his ill younger sister. Unfortunately, Nishiki suffered a Trauma Conga Line soon after and without Kiryu to keep him level he became more and more ambitious, and by the time they meet again after Kiryu makes parole Nishiki is not above trying to frame and/or assassinate Kiryu if it means he'll continue to rise through the ranks of the Tojo Clan.