Afro and Jinno in Afro Samurai. Interestingly, it's the protagonist, Afro, who "turned evil" — Everything Jinno does is to protect those at Swordmaster's dojo, while Afro is motivated purely by revenge.
Nina and Arika's humorous Odd Couple relationship in Mai-Otome disintegrated due to a number of...misunderstandings, to put it mildly. They started out as rivals in academics as a result of Garderobe only allowing a limited number of students to advance, but over time, the rivalry is over the love of Nina's adoptive father Sergay. When it turns out that Sergay is anonymously supporting Arika, Nina becomes Nagi's Otome and tries to kill Arika.
Spike and Vicious in Cowboy Bebop, although this is a probably an inversion - Vicious didn't turn evil (it's pretty clear he was always a sociopath) so much as Spike turned good. Or at least turned neutral.
Actually, it is implied that Vicious didn't use to be this much of a sociopath, and Spike's betrayal pushed him over the edge.
Lelouch is an inverted form of this trope. Lelouch's philosophical disagreement with Suzaku over whether to destroy Britannia or fix it from within leads to the two childhood friends at gunpoint in the Season 1 finale. In R2, Suzaku and Lelouch come to the conclusion that there's a better option than destroying Britannia or fixing it from within doing both.
Temporarily reversed in R2 when Suzaku is seen to be playing for Charles but instead of them turning on each other, Suzaku eventually joins forces with Lelouch to put an end to the wars altogether.
Megumi from Gate Keepers who turned traitor because she envied Ruriko's seeming superiority.
And speaking of Yu-Gi-Oh, Yu-Gi-Oh! 5Ds's Kiryu Kyosuke was the lead's best friend as the leader of Team Satisfaction, but he got too...excited by the whole conquering-Satellite thing. When he tried to duel all of Security (the police force) and ended up killing one of them, leading to a situation where he believed that Yusei had turned him into the police, Yusei's and Kiryu's friendship got shot to hell. Kiryu died, which ironically meant that he got better and everything else got worse at the same time. And by everything else, that means he got so Ax-Crazy you could make a case of him reaching critical, Dilandau Albatou levels.
Also from 5D's, Jack Atlus. Prior to the series setting he was also a member of Team Satisfaction, and a friendly rival to Yusei. He grew more and more distant from his friends following the break-up of the team, and eventually accepted an offer to become King in return for betraying Yusei by stealing his D-Wheel and Stardust Dragon, which he accomplished by endangering Rally's life. He's by far a less extreme example than Kiryu, though, as this was the last truly evil action he took, and once Yusei settled their score he quickly became a protagonist and sought redemption in the second season.
Kenshiro and Shin from Fist of the North Star were friends from different martial art schools who were in love with the same woman until Jagi, Kenshiro's jealous brother, swayed Shin into betraying Kenshiro and taking Yuria for himself.
In the anime version of Dragonaut: The Resonance, Kazuki and Jin started out as friends. Jin was forced to drop out of the program they were both working in during an accident which ultimately resulted in Kazuki's Dragon rejecting him and accepting Jin. When Kazuki finally gets a Dragon and ends up fighting Jin, he gets badly scarred, causing him to hate Jin even more.
In the Ice and Snow arc of D.N.Angel, Kyle, Elliot's best friend does this because of the Two Guys and a Girl scenario. He's eventually doomed a la Rime Of The Ancient Mariner to tell the story so others don't make his mistake after he causes BOTH of their deaths. Sort of. It's complicated.
Kenichi: The Mightiest Disciple works this in, but it's among the many things Kenichi has *forgotten* since his childhood. Big Bad Odin is really a childhood friend who made a promise with Kenichi that they would both get stronger...before insisting on fighting over a pin Kenichi received. Odin reveals that it was the loss in that childhood fight, coupled with the humiliation of being offered the pin afterward anyway, that set him on the Path of Asura.
Enchu for Muhyo in Muhyo And Roji. Initially, his grades were far superior to Muhyo's because he put in effort while Muhyo did not, but as Muhyo's talent awoke, he began surpassing Enchu. This was a particularly sore spot for Enchu, as he hoped to become an Executor to support his sick mother. Losing the position to Muhyo shortly after losing his mother broke Enchu, and resulted in his Face-Heel Turn. It turns out that Teeki invoked this trope to find a powerful body to take as his own, and killed Enchu's mother to turn him evil.
Cardfight!! Vanguard loves this trope. In Kai's backstory, he becomes friends with Ren Suzugamori after transferring to the same school which Ren attends. Ren soon obtains Psyqualia and begins losing his sanity before the start of the series, while Kai, having no idea how to save his friend, decides to leave Ren behind and move back to the town where Aichi lives. Kai walking out on Ren actually made things worse. Then, we move on to the beginning of the story proper, where Kai has taken up a cold and distant approach in an attempt to keep history from repeating. He does end up joining a team with Aichi Sendou, only for Aichi to start developing Psyqualia and losing his sanity as well. Thankfully, they both regain their sanity by the end of the first season.
The "rivalry" part of their relationship (in addition to the blinding, obsessive vendetta) is still in full force, as the reasons for all of Doom's evil plans to Take Over the World, gain cosmic power, topple governments, decimate the superhero community, and rain death and destruction upon all oppose him can eventually be traced back to his desire to get one over on his old college pal. (He'll even delay killing Reed to make him admit Doom is better than he is.)
In theSpider-Man comics, Eddie Brock was a reporter that developed a grudge against Spiderman when the latter apprehended Sin-Eater. Brock had been interviewing a compulsive confessor and lost his job in disgrace. In addition to his job loss and inability to find a new one in journalism barring tabloids, he was also diagnosed with terminal cancer, so when he encountered the symbiote (which had tracked Brock by his negative emotions and the cancer upon which it feeds) in the church where Spiderman had abandoned it, he was more than willing to use the symbiote to get the revenge he craved... armed with the knowledge of Spiderman's secret identity and calling himself Venom in reference to the types of articles he was paid to write. He ultimately reformed and gave up the symbiote knowing that without it he would eventually die of the cancer ravaging his body. Except... the trace of the symbiote in his blood fused with white blood cells and created the menacing Anti-Venom, who, while not as evil, and fights villains like the current Venom Mac Garnagan (AKA: The Scorpion), still holds a grudge with ole Spidey. Oh well.
This was inverted in the 70's Marvel Universe comic Shang-Chi, Master of Kung Fu. As an acolyte of the despotic ambitions of his father, Fu Manchu, Shang-Chi was always just a little better than his childhood friend, Midnight. But in this case it was Shang-Chi who turned, rejecting his father's evil and leaving the organization. Naturally, his old friend got sent after him. Said friend added a righteous sense of betrayal to his previous resentment.
Legion of Super-Heroes: Some versions of Lightning Lad's relationship with his brother Mekt (Lightning Lord) go this way.
Subverted in ElfQuest. Rayek starts out as Cutter's rival of sorts. Several times in the story, he appears to be about to turn evil, but despite his screwups, he never does, at least, not permanently. Rival Turned Heel-Face Revolving Door?
The Superman villain Conduit was once a childhood rival and friend of Clark Kent but tried to kill him years later.
Tony Stark gets one for a short time in the form of Tiberius Stone, who's implied to have been a sociopath from the beginning, just very good at hiding it. "Short time" because by the end of the arc, Stone has been left comatose, and tends to be forgotten by later writers.
In Wolverine: Origin, Dog starts out as James Howlett's friend and possibly secret half-brother, but eventually they drift apart, and Dog even kills James' puppy. Years later, Dog tracks him down and they fight, and it's possible Dog became Sabretooth later on. This one also works for Two Guys and a Girl, with James, Dog, and Rose.
Catwoman: Cassandra Cartland grew up in the same orphanage as Selina Kyle where Selina rebuffed Cassandra's attempts at friendship. Cassandra grew bitter and resentful and, after learning that Selina had become Catwoman, created her own villainous identity of She-cat and attempted to frame Selina for crimes she committed.
In Shaman's Tears, Thom Broadarrow was a childhood rival to Joshua Brand. He was always jealous of Joshua, especially when Joshua got the girl they both wanted, then abandoned her to leave the reservation; something Thom never managed. When Joshua, returned to the reservation and was granted the power of Wakan Tanka, Thom thought it should have rightfully be his. His jealousy ultimately led to accident that transformed into a monster.
Nami and Elly in Pirate Meets Gebler when the latter used the Drive to not only fight Fei, but also the former. They got better after the former got the latter to snap out of this by threatening to drop herself off the canyon..
Eddie Brock is portrayed this way in Spider-Man 3, as fighting for Peter's position at Jameson's office. The Rival/Evil Turn comes after he tries to cheat his way ahead and Peter exposes him; Peter, wearing the Venom suit, goes to a local chapel seeking repentance, while Eddie goes to the same seeking revenge; the church bells ring, disturbing Venom sufficiently that Peter is able to fight him off, and Venom drops down a few stories to take over Eddie, who is a much more willing host...
While he and Ip Man never became friends, Jin descended from martial artist rivalry to becoming a bandit and later selling Ip Man's last known whereabouts to the Japanese. He does a Heel-Face Turn back by the second film.
The 2002 film version of The Count of Monte Cristo with Guy Pierce and Jim Caviezel differs from the book by making Dantes and Mondego good friends with a friendly rivalry before Mondego betrays Dantes out of jealousy.
The escalation of this effect drives the plot of The Prestige. Robert Angier and Alfred Borden are both amateur magicians working fairly amicably as audience plants for another magician. However, when Angier's wife is killed and Borden appears to be responsible (it's possible that he's entirely innocent) Angier devotes himself to destroying Borden by any means including sabotaging and stealing his tricks as well as killing himself dozens of times over just so he can eventually frame Borden for murder and take his daughter. Borden remains far from innocent though and retaliates by constantly ruining Angier's chance at fame and stealing his mistress.
Like his comic counterpart, Iron Man had his fair share of evil rivals. The first was Obidiah Stane who, in the Marvel Cinematic Universe was an in-company rival to Stark Enterprises as opposed to the head of his own company. He eventually turned evil and tried to kill Stark. In the second film, Justin Hammer was a business rival who ended up releasing an international criminal who had previously killed many people and tried to kill Stark, in order to defeat Stark Enterprises.
Overlaps with Love Makes You Evil in The Sorcerer's Apprentice. Maxim Horvath was a friend of Balthazar Blake and was in love with Veronica Gorloisen, with all three being Merlin's apprentices. Veronica chose Balthazar, resulting in Horvath deciding that he no longer cared about anything and would willingly help Morgana destroy the world and is not shy about trying to kill Balthazar. Centuries later, Balthazar is surprised that Horvath still feels bitter about Veronica's choice.
James Potter and Severus Snape never got along at school, made worse by the fact that Lily, Snape's childhood friend who he was thoroughly in love with, ended up marrying James, while Snape turned evil. Snape turned back to the side of good because Lily was in danger, and stayed on the good side because his previous evil actions caused her death.
Draco Malfoy, after having been The Rival for five books, joins Voldemort's side and becomes significantly more dangerous in the sixth book.
Saruman in the Lord of the Rings turned to the Dark Side because he was jealous of Gandalf having received the Ring of Fire from Círdan upon their arrival in Middle-Earth.
Happens with Brutus in Con Iggulden's Emperor series. He starts out as Caesar's loyal blood brother, but as time goes by he grows increasingly jealous of his friend's success as a general and a politician. Eventually he leaves Caesar to fight for Pompey in the civil war, and tells Caesar later that he did it because he wanted to be someone without having to stand in the shadow of the great Caesar.
It seems as if half the Forsaken in The Wheel of Time had been very talented and effective good guy generals and Aes Sedai who could not stand that their rival Lews Therin Telamon (whom they were at least fairly friendly/neutral colleagues with at one stage), The Dragon, was better than them, thus turning to the Dark One (Not the case for other Forsaken like the sadistic, and unparalleled healer, Semirhage, or The Philosopher Ishmael).
Played most straight with the Forsaken Demandred. His entire life can essentially be described as "almost as good as Lews Therin". He was born a day later. He was ever so slightly weaker in the Power, and ever so slightly less proficient at its use. He was slightly shorter. He was slightly worse looking. Ilenya chose Lews Therin over him. He was passed up for becoming the Amyrlin Seat in favour of Lews Therin. Lews Therin's deeds in the War of Power were slightly more impressive. So on and so forth. Characters from the time who think back say that in any other time, in any other place, he could have been the greatest hero of the age, but he had the misfortune to be born into Lews Therin's time. He fell to the Dark primarily out of jealously for Lews Therin, and to this day is described as the Forsaken with the most hate(particularly for Lews Therin, but in general as well). Not the most anger, not the biggest ego, not the most sadistic. He just hates.
Pasha in Doctor Zhivago, though he was Lara's friend (and thus more of a romantic rival for Yuri).
Septimus Heap: Simon Heap became an antagonist after Septimus got the ExtraOrdinary Apprenticeship.
Breezepelt from Warrior Cats was originally the rival to Lionblaze and Jayfeather, but he was friendly enough to team up with them on occasion. However, in Omen Of The Stars he is recruited by the Dark Forest and turns homicidal.
In "Song of the Lioness," Alex becomes this to Alanna
The Master and The Rani from Doctor Who are two examples as well.
Hiro Nakamura and Takezo Kensei/Adam Monroe of Heroes.
Although they didn't have much 'rivalry' initially; Hiro behaved more like some kind of Manic Pixie Dream Girl very much minus the 'dream girl' part.
In Kamen Rider 555, the two main characters, Inui Takumi and Kiba Yuuji, switch back and forth between friends and enemies, not once but several times. Eventually, Kiba does a Face-Heel Turn and becomes an all-out villain, but in the last episode he returns to the Light Side and does a Redemption Equals Death.
The early seasons of Smallville dealt with Clark and Lex's relationship this way.
24's Jack Bauer seems to have an endless string of these, particularly Christopher Henderson. He and Tony both got to see the sights of Nina Myers.
Street Fighter: Sagat turned to crime and hatred after losing a fight to Ryu (and getting hideously scarred in the process - In his defense, Ryu did attack him with a cheap shot, while under the influence of the Satsui No Hado). He turned good again at the end of Street Fighter Zero/Alpha 3.
One of the plotlines of Breath of Fire: Dragon Quarter mostly follows this trope. Ryu and Bosch start the game, while not necessarily as mutual friends, at least as co-workers. When Bosch starts observing (and once ends up on the wrong end of) Ryu's suddenly-manifested dragon powers, he jealously begins resorting to increasingly twisted means to match that power, including getting a freaky cybernetic body and, eventually, gaining similar powers from an evil dragon, after it sensed his selfish motives.
In the backstory for City of Heroes / City of Villains, the men who would become archnemeses Statesman and Lord Recluse were once very good friends. When they drank from the Well of the Furies, one became an Incarnate of Zeus, got lightning powers, and named himself Statesman, while the other became an Incarnate of Tartarus, grew spider arms, and named himself Lord Recluse. One of them became an evil overlord.
Ryudo and Melfice in Grandia II were in fact brothers, but like everybody from their soon-to-be-cursed village very competitive. Melfice turns out to be possessed by the Horns of Valmar, forcing the brothers to fight. Additionally, Ryudo meets up with his childhood friend and rival who has to admit he's become soft and Ryudo is currently a better swordsman than him, at least until he starts training again.
Cecil and Kain in Final Fantasy IV. Kain was being (at least partially) mind-controlled, and he repents eventually, but his jealousy over Cecil (mostly regarding Rosa) makes him that much easier to control.
EarthBound: Ness and Pokey, though they weren't ever really friends. (Ness is on better terms with his brother.) Understatement. The guy starts out cowardly, bitchy, and self-centered (you see the type in media being self-aggrandizing when the danger has passed). Things go downhill with him from there. Oddly, he doesn't seem to know what he wants to be as far as Ness goes. In Mother 3, you can find Ness' yo-yo in Porky's playroom in the thunder tower, it's named "Friend's Yoyo".
Before that, Hugh Baldwin in Castlevania: Circle of the Moon. He's the son of his and Nathan's master, and is jealous enough of Nathan getting the Hunter whip that he turns evil and is fought as the second-to-last boss.
Straybow in the Medieval chapter of Live A Live; jealous of his childhood friend Oersted's fame and success, he ultimately orchestrates a plan intended to thoroughly disgrace and break him. Whether he intended it or not, he ultimately ends up causing Oersted to undergo a major Face-Heel Turn, declaring that Humans Are Bastards and becoming the game's Big Bad. So, in the end, both rivals turn evil.
Songi starts as Gala's rival in Legend of Legaia. He quickly turns more and more depraved and villainous in his journey for greater power.
Revya becomes this to Danette during the Demon Path in Soul Nomad.
Steiner is blatantly set up for this in Luminous Arc 2, although it's worth noting that from his viewpoint, the hero has turned evil (having collaborated with several criminals, one of whom was an accomplice to a mass murderer.) The two go through the standard "redeem each other or kill each other" schtick before Steiner sacrifices himself to kill a greater villain.
Jake from Ape Escape is turned into Spike's enemy by Spectre' brainwashing.
Dirk in Suikoden Tierkreis is a mentor turned evil—not an Evil Mentor, but a formerly good mentor who succumbs to a combination of fear, suspicion, and jealousy after his proteges a): suddenly acquire magical powers they never worked to earn, and b): start to remember things he's certain never existed. He starts out just trying to snap them out of it, but winds up barely coherent.
Peter Parker and Eddie Brock in The Spectacular Spider-Man were childhood friends, but over the course of the series they drift further apart, as Eddie's grudge becomes greater, leading to their destined battle as Spider-Man and Venom.
In Transformers Prime Optimus and Megatron were once friends (hardly rivals) as Orion Pax and Megatronus while they worked together to progress civil rights on Cybertron. It didn't end well. Optimus still thinks he can bring his old friend back until towards the end of the first season.
Splinter (Hamato Yoshi) and the Shredder (Oroku Saki) as seen in the 2012 Teenage Mutant Ninja Turtles. They were once good friends until they both fell in love for the same women, Tang Shen. Their rivalry for her love soon turned into hatred, which ultimately killed Shen and Splinter's infant daughter.
Jack Fenton and Vlad Masters although the former is unaware of this case until the Grand Finale.
Tucker becomes this in "What You Want", going too far with his ghost powers and attempting to outshine Danny about it. He eventually gets better from it.
The Adventures of Jimmy Neutron: Boy Genius has an unusual case in the episode, "The N-Men". For this example, the titular character is the rival turned evil being angered on how Cindy keeps on overshadowing him. This eventually sets him off and he goes on a rampage. Ironically, the only person who can calm him down was Cindy, who he was originally mad at.
Morph becomes this in X-Men season 2. Wanting revenge on the X-Men for leaving him to die, his acts in the beginning of season 2 includes posing as a reverend to lead Scott and Jean away for Mr. Sinister to get them, locking Beast away in the Danger Room and increase the danger, and getting Storm shot while posing as a police officer. Then, it's a subverted when he has a Split Personality doing all this. He eventually gets better from this by the season 2 finale.