Blake's 7: The Federation has never not done this, which raises the question of why anyone continues to betray the rebels to them (or, later, to Servalan acting alone). Sure shows how evil the bad guys are, though. The traitors are killed, except in one case where Servalan just took his only spaceship and marooned him. Sometimes the traitors shout things like "I gave you Avalon [a rebel]! I gave you Avalon!" or "I served you well!!"
Bugs: Lampshaded, with it not being uncommon for the good guys to explicitly point out to people being blackmailed by bad guys (typically in exchange for a hostage release) that villains never carry through with their end of the bargain, and will either kill them anyway after they're done, or continue to ask for things. Also played straight, where, after asking his partner to kill his daughter, John-Daniel shoots his partner on account of "How could I trust a man who'd kill his own daughter?" and spends the rest of the season working with the daughter.
The Caesars: At the beginning of "Caligula", Macro smothers Tiberius to help Caligula succeed to the Imperial throne. At the end of the episode, Caligula forces Macro to admit that he was technically still serving Tiberius at the time, and concludes that if Macro would betray the Emperor he served once, there is no reason why he might not do so again, and that he and his wife must therefore do the honourable thing and commit suicide. (In real life, Caligula pretended to offer Macro the governorship of Egypt, only to have him murdered as soon as he boarded the boat for Alexandria.)
Newly elected PM Harold Saxon begins his first cabinet meeting by calling his ministers traitors for abandoning their parties to support him once they saw the votes swinging his way. He ends it by gassing them to death.
Done in "The Sontaran Experiment", when Vural betrays his party to Styre to save himself from experimentation. Styre goes back on his end because he doesn't want to deal with "a traitor to his kind." However, Vural's death is a Heroic Sacrifice, when he saves the Doctor's life.
In "Daleks in Manhattan", Mr. Diagoras helps the Cult of Skaro gather test subjects and build the equipment needed for their "Final Experiment". The Daleks reward him by using him to make the first of their Dalek-Human hybrids. Of course, considering that they had previously complimented Diagoras by telling him that "[he] think[s] like a Dalek", it's possible they actually saw this as a reward.
This tends to be how the Daleks act often, not that it isn't quite predictable since they view all non-Dalek life as pests to be exterminated. A traitor can buy themselves some time, but the Daleks will get rid of them sooner or later.
In The Runaway Bride Donna's fiance Lance is secretly working with the Racnoss Empress, and has been manipulating Donna so the Empress can use her to revive her sleeping children. As soon as the children are ready to awaken, the Empress feeds Lance to them; apparently she doesn't approve of males who mistreat their mates.
An Averted Trope in "The Seeds of Doom." When dissatisfied World Ecology Bureau pencil pusher Richard Dunbar puts Eccentric Millionaire Harrison Chase on to the existence of the Krynoid pod so Chase can add it to his private collection, he wants a lot of money in exchange for the information. It seems like he'll instead kill Dunbar when he comes to collect his money later, but Chase actually pays him off as he'd promised and lets him go unharmed.
Emmerdale: Sadie King blackmails business rival Zoe Tate into selling her property over to her. This is achieved by Sadie, knowing that Zoe is a lesbian, getting Effie Harrison, who was employed by Zoe as a nanny, to firstly pretend that she has romantic feelings for Zoe and secondly to persuade Zoe to go on the run with her. Unfortunately for Zoe, who was in a vulnerable state and worried about an upcoming court case, she falls for it and when she goes to meet Effie she finds Sadie there instead. After this is done, Effie asks Sadie for the payment that she promised, and Sadie throws some loose change on the floor calling it "30 pieces of silver" and telling Effie that "Zoe is more of a woman than you'll ever be", leaving Effie to claw for the coins in the dirt.
Rygel sells out the rest of the cast in the first-season finale, but quickly changes his tune when Crais tells him in no uncertain terms what's in store: Scorpius will use Rygel to catch Crichton, but then he'll have him killed slowly "to show everyone what he thinks of traitors". Luckily for the little toad, Crais has a complex double-betrayal of his own in mind, and one side effect is Rygel's safe return to Moya.
Scorpius later tries this again with The Mole in "Look at the Princess," ordering Braca to Leave No Witnesses after she turns Crichton over to him. In this case, however, Crichton kills her himself before Braca can.
Happens to Jayne in the episode "Ariel" when he tries to turn Simon and River in to the Feds: the Feds, led by Agent McGinnis, arrest all three of them because McGinnis wants to keep the reward money for himself. The result is that Simon and River don't realize that they were betrayed by Jayne, while Jayne is forced back to his original side. When the three of them are taken to a holding facility Jayne proceeds to take out the Feds guarding them and free them both. When Mal finds out what Jayne did he's ready to have him Thrown Out the Airlock. On McGinnis' side, when the Hands of Blue discover that he and his men have spoken with River in the first place, they promptly kill them with their fancy sonic weapon.
In "Heart of Gold'", one of the female prostitutes at the brothel opts to side with the scumbag misogynist villain Rance Burgess. Her reward is to be subjected to a speech on how women are subservient to men, before being forced to blow him in front of a crowd of people. At the end of the episode, she is kicked out of the brothel and forced to go back to town with Burgess's men.
Game of Thrones: In the second season, Robb Stark offers the Ironborn who invaded Winterfell amnesty and safe passage home if they turn over Theon Greyjoy, the nominal heir of their king and the man who led them there. They promptly do so...and are rewarded by falling into the hands of Roose Bolton's son Ramsay Snow, who ignores Robb Stark's command and has them all flayed alive for his amusement.
LOST: Subverted Trope: in the second season finale, Michael sells out his fellow lostaways to Ben in exchange for his son and a boat off the island. The boat pulls away, Ben's eyes follow it ominously, and he even mutters "Bon voyage"... and nothing happens. Well, not right away...
Mission: Impossible: In "Reprisal", an orderly has been helping the villain escape the maximum security mental hospital to commit murders. When the orderly gets cold feet and demands more money, the villain promises him "a big payout" when he makes his final escape. The "payout" is being murdered.
Monty Python's Flying Circus: A comedic (albeit sociopathic) example in the "How Not to Be Seen" sketch: "When we called at their house, we found that they had gone away on two weeks' holiday... However, a neighbor told us where they were." The house that they're in blows up. Cut to a Gumby. "And here is the neighbor who told us where they were." He blows up.
NCIS: Los Angeles: Years ago Sam was kept safe from the Taliban by an Afghan elder's Sacred Hospitality and was able to get the man and his nephew into the US. What Sam didn't know was that the Taliban killed the elder's family and the nephew's been holding a grudge against Sam. After Sam is captured by the Taliban the nephew hopes he and his uncle will be released but the Taliban recruiter reveals that they're both going to be executed: the elder for defying the Taliban and the nephew for betraying his family. The team saves the day but the nephew can't let go of his grudge and his uncle winds up killing him to save Sam.
Former rebel Brian who gives up Miles and Nora's ultimate destination in "Ghosts" gets the same treatment. The Monroe Militia, for all of its other deficiencies, seems to be Dangerously Genre Savvy when it comes to dealing with defectors and informants.
Averted Trope with Mia Clayton, Nora's sister, by Sergeant Will Strausser, of all people in "Ties That Bind". In exchange for the power pendant, he spares both their lives.
Averted Trope with Tom Neville, who defects to the Georgia Federation with his wife in tow after "The Song Remains the Same". President Kelly Foster puts him in charge of working with Miles Matheson in "Home" and "The Love Boat". Unfortunately, sparing Tom Neville proves to be a mistake, because in "Children of Men" and "The Dark Tower", he betrays them by taking over the Monroe Republic and he intends to take over the continent.
Stargate SG-1: Any Jaffa who betrays his Goa'uld master is branded as a Shol'va (literally, "traitor") by all other Goa'uld, despite their feudal nature. This is largely because the Goa'uld rely heavily on their Jaffa to maintain control of their empires; any Jaffa that willingly betrays their master is a threat to the balance of power. Especially given that it could undermine the Goa'uld claim to being gods.
Star Trek: The Next Generation: One episode played with this. Riker was made second in command of a Klingon ship as part of a cultural exchange program, and was made to swear a loyalty oath to the Klingon captain. Events happen that resulted in the Klingon captain declaring that the Enterprise attacked them, and so they were going to attack the Enterprise. The Klingon captain then demanded that Riker give him the access codes to the Enterprise, which Riker refused to do, due to his pre-existing oaths to Starfleet, but he was still willing to participate in the attack as part of the captain's orders (well, not really). The Klingon captain proceeded to tell him that if he had betrayed his original Starfleet oaths, he'd have had Riker executed as a traitor.
Sometimes people who flip and join other alliances end up voted out first when their target(s) are gone. There are several reasons for this: One that you would probably not want someone who you know would flip around, two because they often end up the third wheel in an alliance and the third wheel often goes out.
Shambo, did you really think Russell was going to take you to the finals, knowing the jury would be full of Galu sympathizers, and that he had two other people he thought were more worthless?