Tony Almeida in the seventh season of 24. He betrays Emerson's group, which he claims he had actually been loyal to at one point for serving as a deep cover agent for Bill Buchanan, then betrays the FBI by killing Larry Moss after thwarting Juma and Hodges' plans in favor of the masterminds behind the conspiracy, all so that he can meet their leader face-to-face and kill him.
A Discovery of Witches has Domenico, who constantly betrays and sells out his benefactors, to the point it becomes a Running Gag that he negotiates or re-negotiates his price in every other scene.
In Alias, Mr. Sark was known for his "flexible loyalty".
When Angelus is unleashed he discovers a demon that blocks out the sun. Vampires rule the day as well as night, and when Faith tries to stop him he beats her. Angelus had won, and the demon is about to kill her, then Angelus picks up the Villain Ball and grabs it hard by destroying said demon. Not for power, not so he could kill Faith himself, just because (he also doesn't like being told what to do, and his intention was that the demon would only weaken her). Unfortunately for him, this undoes the sun-blocking spell, something he had hoped was "just Angel's retarded fantasy".
Connor. He may have picked up the condition from Holtz.
Malcolm Merlyn from Arrow is the poster boy for this trope. Lampshaded in one episode with Felicity claiming "He is a mass murderer who has lied to us so many times it should be a drinking game."
Baltar from the classic Battlestar Galactica. He betrays his own people to the Cylons, then turns around and betrays them in the last episode ''The Hand of God’’.
The Grant clan from Big Love are all like this. By this point, you need one flow chart just to figure out who's related how to whom (living on a xenophobic polygamist compound where wives get swapped around at the whim of the Big Boss can do that to you) and at least three more to keep track of who's currently trying to have how many of said relatives jailed, killed, or terrorized into submission (and who's just in it for the book deals).
Birds on the Wing: Despite working together to con others, Charles and the girls can't resist trying to con each other too. This almost ruins their con pulled on Mr. Uskudar in "Mr Uskudar's Con Nearly Goes Wrong", while Charles' schemes in "Charlie Tries to Con Elizabeth and Samantha" lead to the girls finally getting one over him in "Charlie's in Geneva, Girls are in Tokyo", when they run off to Tokyo with his money.
This is Spike's role, at least until he falls in love with Buffy.
Xander: Spike's working for Adam!? After all, we've done — nah, I can't even act surprised.
Warren sold out his robot, his girlfriend and both his best friends.
In the second season paintball game on Community, Chang continually teams up with and betrays almost every pack of combatants who will take him, even if they're winning.
In the fifth season, Chang joins the "Save Greendale" committee, only to betray it in the final episode. Absolutely no one is surprised, to the point where Chang's betrayal was apparently on their to-do list.
Dean Pelton admits in the second season "Conspiracy Theories and Interior Design" that he can't keep track of all the lies and just teams up with whoever suggests it.
Continuum gives us Matthew Kellog, who over the course of four seasons manages to not only betray every faction multiple times, but also, thanks to the magic of time travel, backstabs himself, with a plan to travel back in time and steal his own kidneys.
Omen on Dark Oracle suffered from this as part of his Heel–Face Revolving Door. Somewhere in the backstory he betrayed Doyle and was turned into a frog as a result. When he reappears, he romances Cally, only to betray her and use her as a pawn against Doyle and Lance. He later promises Cally that he will rid her of Blaze and Violet if she returns his humanity. She does so, but Omen's attempt at killing them only makes them stronger and results in his imprisonment. After being freed by Vern he offers Vern a chance at revenge on Lance, only to betray Vern by taking it too far and trapping Lance in the mirror world. When Cally comes to him for help he betrays her by swapping out Lance for Blaze who he was really working for and helping them set up a curse that will eliminate Cally as well and let Violet escape. Finally, he betrays Blaze & Violet, helping Cally free Lance and dying in the process. Phew.
The Daleks frequently betray any and all individuals/species that think the Daleks are working for them, almost always with a cry to "EXTERMINATE!" the betrayee.
Pretty much lampshaded in "Victory of the Daleks", where a group of impure Dalek refugees manage to activate a device which makes shiny new model "pure" Daleks, which then proceed to immediately exterminate the old Daleks, who die willingly, accepting they are inferior.
"Genesis of the Daleks" makes it plain where they got it from: their creator, Davros, also proves to be a good example of this trope. The fact that he does not see their betrayal of him coming adds a nice layer of irony to the episode and the fact that they have proceeded to repeat this betrayal multiple times, and that each time he has failed to see it coming, just adds to the deliciousness of it.
"The Daleks' Master Plan" is an extreme example, where the inevitability of the Daleks' betrayal of Mavic Chen — the Daleks announce it to the audience and to each other every couple of episodes and even Chen seems to know they're going to do it — leads to much of the tension being "how long can Chen keep them convinced he's still useful?" Mavic Chen is already a traitor, being the most powerful man in the Solar System already but planning to gain more power by betraying humanity to the Daleks. This is pointed out by one of the Daleks' other allies, as none of them (the Daleks end up betraying them as well) sold out their own people.
"The Power of the Daleks" uses Plot Parallels to make this just as extreme — the Daleks are obviously going to stab the resistance and Lesterson in the back, but the resistance is itself backstabbing the current Vulcan government, while all of the members are simultaneously trying to backstab each other.
The Doctor knows about it in "Death to the Daleks", where he explains to the supporting characters that even though they're working with the Daleks, the Daleks are definitely going to try to stab them in the back at some point because that's all they ever do.
The Sontarans. They backstab Irongron, the Vardans, the Androgums, and Luke Rattigan.
Fifth Doctor companion/would-be assassin Turlough originally met the Doctor after the Black Guardian offered the exiled alien schoolboy a lift off planet Earth in return for killing the Doctor. He abandoned and betrayed the Doctor pretty much anytime things got too dangerous, even after Fivey forgave him for the whole, you know, attempted murder thing. However, he always seemed to redeem himself by doing something heroic, especially in his last episode, Planet of Fire.
Whenever the Cybermen make a bargain, they never hold up their end of it. Eleven ruefully lampshades this in "Nightmare in Silver" when he's engaged with a game of chess with the Cyberplanner who's hijacking his mind. If the Cyberplanner wins, he'll hijack the rest of the Doctor's mind and with it gain the secrets of Time Travel. If the Doctor wins, they'll just try to do it anyway. Thus, the game is more of a stalling tactic until the Doctor comes up with an idea.
The Master is also a fan of the tactic, but gets backstabbed almost as many times as they're the one doing the backstabbing. Reaches its zenith in "The Doctor Falls"; Saxon-Master and Missy appear to be working together until she Back Stabs him on the way to his TARDIS and he shoots her In the Back in turn. Both of them realize that they just double-crossed themselves and go to their respective deaths Laughing Mad.
ER's Kerry Weaver, whose modus operandi was basically to pretend to be someone's friend/ally, only to screw them over in order to advance or maintain her career. Mark Greene as well, though in his case, he was usually just too weak and spineless to stand up for someone, rather than actively trying to trip them up.
Sikozu was an embodiment of this trope, her only consistent trait (besides total arrogance) being her capacity for 'sudden and inevitable betrayal'. Close to the end of the fourth season, however, she looked to be outgrowing this character flaw — only for the Scarrans to employ her as a spy during The Movie. Her comeuppance finally came at the hands of her current boyfriend Scorpius, who beat her to a bloody pulp and left her to die.
Grunchlk in the episodes "Die Me Dichotomy" and "Season of Death" qualifies: while overcharging the crew of Moya for various medical services, he quietly betrays them to Scorpius. However, when Scorpius arrives with a squad of heavily-armed Peacekeeper commandos, Grunchlk panics and releases a Scarran warrior from stasis in an attempt to hold the attackers off. And it turns out that the Scarran was also double-crossed by Grunchlk...
During Season 4, Crichton believes that Scorpius suffers from this disorder after he joins the crew. Crichton has good reason to be suspicious of Scorpy, but seems to believe that Scorpius is going to try and backstab him at the most idiotic times: for example, in "I Shrink Therefore I Am" he gives Scorpius an empty rifle just in case any treachery occurs — while they're both stuck on Moya, with all their escape ships disabled, and being hunted by bounty hunters with no interest in negotiations. Lo and behold, Scorpius isn't that dumb. Eventually, Crichton decides that Scorpius can at least be trusted to a certain extent after several incidents where not only remains loyal but even puts his own life on the line to save the day at least twice. Unfortunately, after being used as bait for Talikaa, Scorpius decides he's had enough of his role as Sixth Ranger and backstabs Crichton so masterfully that it takes him two episodes to figure out who was really behind Aeryn's kidnapping.
Rygel. Backstabbing was his default setting. Circumstances seem like Rygel might be somewhat inconvenienced? Time to throw everyone else to the wolves.
He actually uses this trait to his advantage on a couple of occasions, by pretending to backstab the crew when he's actually helping them. And he's such a natural backstabber that even his own allies don't realize what he's up to until afterwards.
Saffron. In "Trash", when she called upon Serenity's crew to help her steal the Lassiter, the heroes cooked up a Batman Gambit to take her down, which took advantage of Saffron's "sudden, but inevitable, betrayal".
Jayne — who repeatedly tries to sell out his own crew for money or to become the captain. Naturally, Mal repeatedly thwarts these plans and generally beats Jayne up or threatens to kill him... but he's a slow learner.
He also betrayed his original crew in order to join Serenity. Why? Well, they were robbing Serenity, and Jayne's share was 7%. Mal offered him 11% and his own bunk. Several shots to the back later, Jayne was Serenity's newest crewmember.
In The Movie, Jayne tries to go behind everyone's back and throw River off the ship once he realizes how much a threat she represents. River advises Jayne of his fallacious reasoning via ceiling-launchedcranial trauma.
And of course, there's Wash's toy dinosaurs: "Ah, curse your sudden but inevitable betrayal!"
Game of Thrones: This is the entire point of the series. (It's all in the name.) Just about everyone wants to rule the Seven Kingdoms of Westeros and is prepared to lie, cheat, and murder their family members and supposed allies to do so. The few (sane) people who don't want to rule have to suffer the consequences.
Littlefinger, who has betrayed his patron Jon Arryn (who made him Master of Coin), Ned Stark to the Lannisters, the Lannisters to the Tyrells and Lysa Arryn — literally everyone he's ever allied with the moment it became more convenient for him. And yet still somehow keeps convincing people to work with him.
The City Watch of King's Landing will always support whoever pays them the most.
House Frey is infamously unreliable because of their passive self-interest: they avoid taking sides unless they have something significant to gain. Most houses are smart enough to assume the Freys will sell them out the moment they receive a better offer.
In-universe House Greyjoy has this reputation due to attempts to rebel whenever they think they have the slightest chance of winning (and even when they really don't).
Balon Greyjoy led a failed rebellion against the newly crowned Robert Baratheon in the backstory, causing his son Theon to be taken hostage by Ned Stark. With turmoil engulfing the realm once more after Robert's death, he rebels again the moment his son returns. And rejects the offer of alliance against the Lannisters that Theon brings from Robb Stark, even though it really would have been to his benefit, instead trying to wage war on everyone.
Cersei politely insinuates that Brienne has this, seeing how she swore to serve Renly, then Catelyn, then Jaime. Brienne replies that she does not serve Jaime. And she only became Catelyn's sworn sword after the latter helped her escape from being falsely accused of Renly's murder.
The Stark family seems to be an inversion of this as multiple members are victims of betrayals which lead to their deaths.
Ned Stark was betrayed by Littlefinger and Janos Slynt to the service of Cersei and King Joffrey, ultimately leading to his beheading.
Rob and Catelyn were betrayed by the Freys and Boltons leading to their deaths well as that of Robb's wife, unborn child, and bannermen in the infamous Red Wedding.
Jon Snow was tricked and betrayed by Olly and Allister Thorn along with other members of the Night's watch, all of whom took turns stabbing him then left him to bleed out. He got better.
Ramsay murders several of his own men to gain Theon's trust, then turns on him. He also habitually breaks promises of safe conduct to enemies who surrender to him — and instead horrifically tortures them by flaying men alive by the dozen and leaving their mutilated corpses on public display. His father has become increasingly frustrated at him for his sheer lack of future planning. In Season 6, it reaches its head when he murders his father, his stepmother, and his newborn brother, all to seize power for himself.
Zigzagged trope with Jaime. This is his reputation. In reality, his killing of King Aerys is far more complex: Aerys told him to kill his own father and was also planning on burning down and killing everyone in King's Landing. Unfortunately, Jaime's pride and pessimism discouraged him from revealing that true reason for killing Aerys. On the other hand, if word were to ever get out that Jaime had brutally killed his younger cousin, Alton Lannister, then he would be known as a kinslayer, the only thing worse than being a kingslayer in Westeros, as well.
Varys is a rare benevolent example. He worked for several regimes over the course of his life and has secretly worked to undermine them whenever they saw they were too oppressive or ineffective and in Season 7, he declares to Daenerys that he is is not loyal to her, or any ruler, but loyal to the people and the realm, and the reason why he backs her is because he believes she is the ruler most dedicated to the people's well being.
The Good Wife: Cary Agos turned on every boss he ever had . Including himself, in a sense, when he went behind Alicia's back at Florrick-Agos.
Season 1 of Gotham recounts the Penguin's rise to the top of Gotham City's criminal underworld, almost entirely by means of this trope. In later seasons, this trope defines his relationship with most of the other villains, notably the Riddler and Jerome. Ironically, the more sympathetic villains on Gotham seem to be more prone to stabbing each other in the back during various villain teamups than the more unhinged villains who just want to destroy everything or cause chaos for the fun of it. For example, while Jerome Valeska is prone to killing his mooks for being annoying or even just for a laugh, he's not motivated by money or power and so doesn't have a reason to backstab any of the major villains he teams up with, unless they betray him first. His alliance with Tetch and Scarecrow doesn't fall apart due to betrayal, even though Tetch outright lampshades the fact that they will all probably end up betraying each other eventually. Jeremiah later allies with Scarecrow, Tetch, and Ra's Al Ghul without having any problems, either, most likely for the same reasons: because they have the same goals and are not actually competing with each other. Oswald, on the other hand, who has quite a few Pet the Dog moments in the series despite being a ruthless mob boss with a Hair-Trigger Temper, continually betrays people for power, money, and even love. So does Barbara Kean, despite generally being A Lighter Shade of Black when compared to people like the Valeska brothers and Scarecrow. Edward Nygma/The Riddler also frequently betrays people over things like love and revenge despite generally having more standards than other villains. While this is counterintuitive, it actually makes sense, as the saner villains are actually capable of forming genuine friendships with each other and therefore more vulnerable to being betrayed.
Halt and Catch Fire: Joe MacMillan is an excellent example. At various points, he betrays Gordon Clark, Cameron Howe, John Bosworth, IBM, Cardiff Electric, his father Joe Sr., Simon Church, and an unnamed IBM employee. Gordon and his father call him out on this. It is a wonder Joe can convince anyone to work with him. Joe is also depicted as being a mentally unstable, egotistical, power-hungry snake in the grass. Yet somehow his charm keeps convincing people to believe in him, even once people realize his MO.
Mohinder Suresh seems to be doing a lot of this, although it's mostly due to him being an atrociously Horrible Judge of Character than any sort of malicious master plan on his part.
HRG, Mama Petrelli, Nathan Petrelli, and Sylar. More backstabbingness than you can shake a knife at.
King Loth of Kaamelott provides the quote on top of the page. Let's see; Season 3: he explains during the end of said season to Sir Dagonet how he associated with Lancelot when he seceded, and how he is the one financing him while trying to recruit Dagonet for a coup against Arthur. Season 4: After openly backing Lancelot's rebellion up during the whole season, he betrays him in a dastardly way in the end (taking back his men; removing every evidence of his involvement) after Arthur rescued Guenièvre, when it is obvious that Lancelot will fail. Season 5: Starts the season by coming all the way to Kaamelott to plead his cause to Arthur. Follows up by trying to unsheath Excalibur from the stone. Finishes the season by trying to zap Arthur with his magic lightning ring. Even Season 6, in addition to the page quote, showed he started plotting against Arthur during the latter's wedding and even went as far as trying to backstab his fellow britton kings while they were fighting the Romans. No wonder he is hated by everyone.
In Kamen Rider Ryuki, Satoru Toujou/Kamen Rider Tiger has this so badly, he's far more dangerous to people he considers allies than his enemies. His reasoning behind this trait is that he wishes to become a hero and, according to his mentor, heroes are people who sacrifice what's important to themselves for the greater good. In Toujou's mind, this translates to killing off everyone important to him to achieve hero status.
In Kamen Rider OOO, according to Ankh, the Greeed as a species suffer from this, though he's an exception because he'll let you know up front he'll betray you if you're no longer of use.
From Kamen Rider Gaim, let's just say that Yggdrasil HRD has some... weird ideas about what quality is needed for its employees. The only named Yggdrasil employee who never betrays anyone is Takatora, who every other member openly considers Too Dumb to Live. Sid tries to betray everyone else at the first opportunity to claim godlike power, Minato openly admits that she'll side with whoever's the strongest, Ryoma plays most of the rest of the cast for saps, and Micchy jumps between so many betrayals that trying to keep track of who he's working for and why ultimately drives him mad when he can't reconcile his constant backstabbing with any kind of logic anymore. Unlike most backstabbers, Micchy is never actually successful, but stringing him along is endlessly amusing, so most of the other villains take a turn making use of him.
Kamen Rider Build lets Blood Stalk pull off backstabbing five villainous organizations, each of whom think that he betrayed all the previous ones on their orders until he reveals that no, he's just the actual Big Bad.
Dr. Smith from Lost in Space, Once per Episode. Although his attempts at betraying the Robinsons were never successful for very long.
Dr. Smith on Lost in Space (2018) appears to have a compulsion to betray people, as she will risk herself to double-cross someone even when there's little to no benefit for herself by doing so. Her propensity to go out of her way to backstab someone for no gain while putting herself at additional risk pushes it into Stupid Evil territory.
Grace's childhood friend/ex-boyfriend Franco on The Mob Doctor, seemingly a double-agent actually working for Moretti against Constantine... until he turns out to be working for the FBI all along.
Ryan O'Reilly is an excellent example, as he changes allegiances purely on his need to survive and manipulates and betrays his supposed allies on a regular basis. His nemesis Nikolai Stanislofsky has the same M.O., and they spend two seasons trying to talk various gangs into killing each other while maintaining a very visible friendship with each other.
Chris Keller aligns himself with Beecher, Schillinger or neither at any given time depending on his whims. At one point he gets a literal, almost fatal stab in the back in return.
Simon Adebisi betrays literally all of his allies as he gradually takes over Em City. By the time they all catch in that they've been duped, it's far too late to retaliate or do anything about it.
Person of Interest features this with the organization of Dirty Cops HR. This is in sharp contrast to mafia boss Elias who has extreme loyalty to and from his people. His largest problem with them is that they are "oath breakers." Even when HR is plotting to kill him, he isn't bothered by the fact that they are plotting to kill him, only by the fact that they betrayed others to do it.
"Matsya Nyaya"note law of the fishes features this concept quite heavily, with virtually everyone in the episode backstabbing someone else. It all starts when an armored car courier tries to rob his own vehicle before running afoul of HR.
Psych: Shawn's uncle Jack offered fifty percent of a fortune in Spanish gold to his nephew. And his partners. And his other partners. And the guy at the muffler shop. And the guy at the Chinese restaurant. Yes, that's three hundred percent.
Shawn and Gus frequently betray one another. In fact, there's a moment of selling out between one or both of them in almost every episode.
In Queen of the South, Guero D'Avila has betrayed nearly every outfit to which he's ever been attached. Naturally, when he joins Camila's group, Camila gives John a standing order to shoot Guero if it looks like he's going to switch teams again.
Titus, who sees himself fit as the master of the Templars because he’s well aware that his superior Petruchio doesn’t have the guts to actually lead his soldiers into battle or fight his foes face-to-face.
Kurdoglu, Suleyman Shah’s younger brother, who intends on forcefully snatching the chieftain’s position with support from Karatoygar and Petruchio. He does manage to take over the clan near the end of the season, but since his elder sibling isn’t dead yet, he only gets to remain the Bey for a short time.
Vasilius displays this attitude toward the Tekfur of Karacahisar, intending on murdering the official and using that power to launch a campaign to massacre the Kayis and other Muslim societies in the area once he gets the opportunity.
Aydogmus Bey sports a variation of this toward Gunalp Bey, fully intending on storming into the Kayi tribe while Gunalp isn’t around, though whether or not he truly intends on overthrowing his supervisor is never made clear.
In Season 5, Dragos holds this attitude toward Tekfur Yannis. Tragically, the latter only begins to realize how vulnerable he is to Dragos once Lais (One of Dragos’ minions, posing as Yannis’ top official) proceeds to mortally strangle him, permitting Dragos to take ahold of the former Tekfur’s stronghold.
Brainiac. Unless you are Zod, working with him will end up with you getting backstabbed.
During Season 8, Chloe successfully manages to stab Clark, Jimmy (her fiance and then husband), and villain Davis in the back almost simultaneously. Then in Season 9, she implements Orwellian measures against Clark's will, ostensibly to "protect Metropolis." This includes putting up cameras in Clark's house and casually mentioning that she spends large chunks of the day surfing through everyone in Metropolis's cell phone conversations.
Tess Mercer. By Season 9, there's really no reason for anyone on any side to believe anything she says.
Chloe: At this rate, you're gonna run out of people to betray.
She manages to go the entirety of Season 10 on the Face side of things without actively betraying anyone. The other characters are aware that she is this trope, though, because whenever something bad happens, they accuse her of turning on them.
Here's a fun game. Watch Smallville and have a drink every time someone is or is revealed to be lying to, manipulating, or downright betraying another character. Two drinks if their last name is "Luthor", "Teague", or "Lang".
Stargate, the Goa'uld in our galaxy, the Wraith in Pegasus.
For the Goa'uld, diplomacy between System Lords consists mostly of both sides trying to figure out how the other plans on betraying them. Any alliance lasts exactly as long as it's convenient, with each member waiting for the other to show a moment of weakness they can exploit. This is actually a side-effect of thesarcophagus technology they use to make their human hosts immortal. The Tok'Ra, who avoid using the tech, and act more like symbiotes than parasites to their hosts, get along with each other (and others) much better. They still seem to be secretive and paranoid by nature, though that might be due to being La Résistance.
The Wraith aren't quite as bad since they focus most of their attention on hunting and eating humans. But any time they try to work together, or with others, the hidden blades start coming out.
Star Trek's Mirror Universe is a living example of this trope. Officers assassinate their superiors to get ahead in the ranks. In The Original Series, the Mirror Universe's Pavel Chekov tries to do in Captain Kirk. Enterprise's two-parter Mirror Universe episode had plenty of this too, with Commander Archer betraying Captain Forrest and several of Archer's crew betraying him and each other in turn. Part of what's most amazing about the crew is that they still manage to work together even with some of the people who very cruelly betrayed them. The hideously scarred mirror Tucker, for instance, was still loyal to mirror Archer even after spending four hours in the Agony Booth for a crime he didn't commit and only being released when crew members loyal to Forrest retook the ship from Archer.
Garak (whom Dr. Bashir is constantly trying to decipher) and Quark, whose alliances change based on his own benefit. Quark's family often have to play the same games — which Quark approves. Dukat is another good example, as he is even trusted by his allies even after a third betrayal.
Most Cardassians that appear in the show are either in some kind involved with the Obsidian Order, or on the run from them, which makes Chronic Backstabbing appear like the Cardassians' hat.
Every character that has ever interacted with Crowley has been subsequently backstabbed by him. It's a running trend no one seems to pick up on.
Dean: See, here's the thing when dealing with Crowley -– he will always find a way to bone you.
Castiel, of all characters, comes off as having a severe case of this. It's partly due to him being repeatedly brainwashed and partly due to him having a hard time with the 'free will' and 'doing the right thing' part of decision-making. Meta-wise, making Castiel an enemy or antagonist is a good reason for him to be unable to help the Winchesters.
Rowena, Crowley's mother, betrays and manipulates everyone just like her son. Including her son.
Metatron betrayed Castiel (after specifically getting chummy with him only in order to use him), and arguably betrayed the Winchesters beforehand by deliberately failing to inform them that their current project would kill one of them. He then proceeded to amass followers and betrayed a bunch of them by telling them he had a special, important job for them, which turned out to be going kamikaze by exploding themselves and claiming that Castiel told them to do it. He also arranged for Kevin's death, despite having healed him in a previous episode. He screwed Castiel over again the next season, and although it was blatantly obvious that he wasn't on Castiel's side at that point, the screwing over came right on the heels of repeatedly trying to convince Castiel that they could put the past behind them and be buddies.
Although a case of this ultimately working out in the good guys' favor: the angel Gadreel legitimately helps the Winchesters, then betrays them after becoming The Dragon to a new Big Bad, then gets uncomfortable with what the Big Bad's doing and betrays him, going back to helping the good guys.
Darkonda from Power Rangers in Space actually got himself killed (more than once) because he just couldn't resist screwing over his "allies". His battles against Ecliptor (including a forced Fusion Dance into "Darkliptor") take up a fair chunk of the season, and ultimately Darkonda dies for good in trying to take out Dark Specter, who winds up taking Darkonda with him. Without that, the Rangers may not've had a chance to win.
There was also Psycho Yellow, who teamed up with Psycho Pink to hoard the Rangers' energy (and, it was implied, overthrow their creator Astronema), left Pink to die as soon as their plan backfired, and in general had a habit of leaving the battlefield when things went south.
From Tensou Sentai Goseiger: Do NOT, under ANY circumstances, work with Buredoran. He WILL royally screw you over in the end after You Have Outlived Your Usefulness. Even when Robogog decided to give him amnesia so he wouldn't do so, he later got his memories back and took him and his group down, along with Metal Alice who helped him take over, purely because he couldn't be bothered to fix her up after the Goseiger wounded her badly.
Basco Ta Jolokia from Kaizoku Sentai Gokaiger will betray anyone he works with. Whether it's the Red Pirates, the Zangyack or Sally, his own loyal companion. Suffice to say, he's one of the most dangerous and despicable villains in the series.
Done when funny in Top Gear — the three presenters take it in turns to team up two-against-one, before someone invariably switches allegiance and starts making fun of their former ally.
Charles Brandon of The Tudors may count as this. Season 1 he allies with the Boleyn faction against Wolsey, Season 2 with Cromwell, Chapuys, and the Seymours against the Boleyns, and in Season 3 with the Seymours (and Francis Bryan, whose motive never was explained) against Cromwell. Season 4 he's finally sick of plotting, but he really doesn't like the Seymours. Cromwell also could count as this. He owed his career to Wolsey but still refused to help him in his time of need. He also owed much of his later rise to Anne Boleyn but still frames her for adultery and treason. But, actually, the only one he's truly loyal to is Henry, who he never betrays. This does not save him.
Vintergatan: In At the End of the Milky Way, the Terrible Trio is seen with Chronic Backstabbing Disorder, most egregiously Lennartsson, who badmouths The Professor to The Count behind the Professor's back, then does the same thing with The Count, causing them to mistrust one another and causing the two most prevalent of the Trio to fight amongst themselves.
Both Richard Neville, Earl of Warwick, and George Plantagenet, Duke of York, in the BBC/Starz series The White Queen. First, they join with George's brother Edward to overthrow King Henry VI and place Edward on the throne as Edward the IV. Then they betray Edward in an effort to place George on the throne in his place. Then, when that fails, they ally with Henry's wife, Queen Margaret, to put Henry back on the throne. Of course, George ends up siding with his brother Edward again, against Warwick and Margaret. Needless to say, George betrays his brother again later. All of this is Truth in Television, mind you.
Stringer Bell betrays three people, all of the main characters, and all of them considering him trustworthy. They are, in order, Wallace, D'Angelo (coming and going), and his blood brotherAvon Motherfucking Barksdale. On top of that, Stringer frequently tries to cheat or set up the people he's bargaining with, regardless of whether they're enemies or people who supposed to be on his side.
Jimmy McNulty also tends to backstab his superiors constantly. Regardless of whether those superiors are Obstructive Bureaucrats like Rawls and Landsman or a genuine Reasonable Authority Figure like Daniels. As soon as they get in the way of what McNulty tries to do or thinks is the priority, he will work against and backstab them, regardless of how many good turns they've done him in the past.
Alex Krycek from The X-Files, easily. Good luck figuring out whose side he's on, and if (you think) he's on your side? Well, just don't turn your back to him.
