Kings are historically known to be deadly to the assassins of other kings, in good part from self-interest: they don't want people to start thinking regicide is morally acceptable.
Alexander the Great is known to have done this upon the assassination of Darius III, the late king of Persia, and Alexander's primary antagonist up until that point. Upon catching the assassin, Bessus, he was turned over for a Persian style torture and execution. Don't expect a king to favor regicide.
Darius, on the other hand, was buried in Babylon with full royal honors.
When Pompey lost the Civil War against Julius Caesar, he fled to Egypt, where he was assassinated by the very people he thought would give him shelter. Caesar then travelled to Egypt in pursuit of Pompey, and he was presented with Pompey's head by Ptolemy XIII's Evil Chancellor, Pothinus. Pothinus planned to win Caesar to his side with this action, but this was an Epic Fail because of Values Dissonance. Rather than grateful, Caesar was enraged to see that a consul of Rome and lifelong personal friend of his like Pompey had been betrayed, butchered and insulted by "barbarians", and demanded the beheading of Pothinus (which he was granted).
Roman emperors also tended to follow this policy with their predecessors' assassins. A particularly noteworthy example of this is Emperor Claudius, who followed Emperor Gaius, commonly known as Caligula today. As the Roman biographer Suetonius notes, Claudius ordered all of Caligula's assassins executed, in part because he knew some of them had probably been planning to assassinate him as well.
Speaking of Caligula, he rather hated the way his former co-conspirator Macro kept reminding him of everything he'd done to keep him alive during the reign of his paranoid predecessor Emperor Tiberius and assure his succession to the throne, and eventually either assassinated Macro or forced him to commit suicide (depending on which Roman historian's account one believes). Considering that, according to some accounts, Macro may well have helped Caligula "hurry" Tiberius to his death with a Vorpal Pillow, this could be a kind of delayed version of the trope in play here. While Macro actually basked in his young Emperor's favor in the early months of his reign, Caligula's nervous breakdown and subsequent paranoia had him killing off everybody he deemed a potential threat to his throne, a category for which Macro certainly qualified.
In Snorri Sturluson's sagas of the old Norwegian kings, King Olav has beaten one of the last great pagan leaders, and said leader has gone into hiding. The King promises to place a ring around the neck of whoever brings him his enemy (likely so he can publicly baptize him, a stronger victory for the Christian king). The pagan leader's thrall, Kark, who has fled with him, hears of this and kills his master for the reward... and Olav repays him by cutting his head off, indeed giving him a ring around his neck—of blood, rather than gold.
It should be noted, however, that Kark was not actually killed for being a traitor, but for being a thrall who killed a nobleman. In fact, Jarl Håkon had already warned Kark about this. Had Kark been a freeman, he would have received the promised reward (which in the saga is "wealth and great praise", not "a ring around the neck"). The original aesop of the story was basically "slaves need to know their place", but due to values dissonance modern readers have often preferred to assume that the "noble" king Olav Tryggvasson simply despised traitors. In fact, in the saga, king Olav does not ask for Jarl Håkon to be "brought to him", but says he will reward anyone who will "do him harm" and then crudely mocks the head of the dead jarl after it is brought to him.
He finally united the Mongol tribes when, after defeating his main rival (and childhood friend) Jamukha and forcing him to flee, two of Jamukha's generals betrayed him and brought him to Genghis, expecting to be rewarded. Jamukha was offered to join his side and, when he refused, given a quick and honourable death, while the two generals were boiled alive. Genghis Khan made "do not betray your khan" an universal rule even when the khan wasn't himself.
Targutai, a chieftain Genghis Khan had a serious grudge against, was captured by three men from a subordinate clan and put in a cart to be handed over to Genghis Khan. However, they worried about this happening to them and eventually released Targutai. Genghis approved of their refusal to betray their khan and took the three men into his service.
After having a lot of trouble with the Lusitanian rebel leader Viriathus in Hispania, the Romans decided to deal with him by bribing his own three ambassadors to assassinate him. They promptly killed him in his sleep and returned for their reward. Roman general Quintus Servilius Caepio promptly informed them that "Rome does not pay traitors" and had the men executed.
The trope was set to be repeated, but subverted with Corocotta, who some have identified as a native leader in the Cantabrian Wars (the Roman source merely speaks of "a bandit in Spain"). The Romans offered 200,000 sestercii to the man that brought him to them. Hearing of the reward, Corocotta himself went to the Romans and demanded the reward, and the Romans were apparently so impressed with his balls of steel that they decided to let him go after paying him.
According to ancient Roman historical legend, the Tarpeian Rock (which was used as a place of execution) got its name from Tarpeia, who let a Sabine invasion force into the city in exchange for "what they bore on their arms." She meant their gold bracelets; instead, they killed her by bashing her with their shields.
In Roman legend Camillus was approached by the schoolmaster of a neighboring city, Faleria, with which Rome was at war. The schoolmaster had lured his charges out of the city, and offered them to the Romans as hostages. Camillus, shocked, ordered the schoolmaster to be stripped naked, beaten and bound, and had the schoolboys drive him back to Faleria, where their parents had found out that they had been lured away and were lamenting. When the boys came back, driving their traitorious schoolmaster ahead of them and singing Camillus' praises, the Falerians were so happy that they called off the war and became loyal allies and friends of the Roman people.
The Roman general Sulla, after seizing control of the city, had a number of his political opponents declared enemies of the state. One of these, Sulpicius, was betrayed by one of his slaves. Sulla rewarded the slave for his aid in killing an enemy of the state, and then had the slave thrown to his death from the Tarpian Rock as punishment for betraying his master.
Hungarian tradition has the story of György Szondi, who heroically defended a small fort against the Turkish forces led by Hadim Ali Pasha in the 16th century. The story goes that a tanner from the fort snuck over to the enemy camp and offered to give away the weak points of the fort to Ali in return for "as much gold as his skins can hold". I'll give you one guess how Ali (an honorable man) decided to pay the reward after they won.
In 1306, Kildrummy Castle in Aberdeen was betrayed to Edward I of England by the castle blacksmith, Osborne, in exchange for gold. When the battle was won, the English rewarded Osborne by pouring molten gold down his throat.
Averted in the case of Benedict Arnold. While not especially well-liked by the British, he was (despite his treasonous plot failing) still paid and given a commission as a brigadier general in the British Army. He still died in debt and despised by the few million people he had betrayed though.
However, it should also be noted that Benedict Arnold didn't betray the colonists to become rich or powerful, at least not solely. He was, in point of fact, being treated extremely unfairly by his allies, and turned traitor when he had had enough of it, believing that at least his efforts would be appreciated by the British.
Neither side was lily-white in that. The War Congress passed over Arnold for promotion; Washington, who greatly admired Arnold, managed to get it for him anyway. On the other hand, Arnold was a bit of an arrogant jerk, and definitely engaged in goods dealing that was morally questionable at best, even if there hadn't been a war going on.
Also it should be noted that the British perceived him as abandoning treachery, not engaging in it. He was, after all, switching sides from rebelling against the king to fighting for him.
Nazi Germany (the SS in particular) was even more brutal to Jewish traitors than to other traitors.
In the end, the Nazis "rewarded" all Jews equally: it didn't matter how enthusiastically you collaborated with them, in the end you would end up being shot or sent to the gas chamber. You could win a bit more time and better surroundings for yourself if you turned traitor, however. A big example would be the Sonderkommando, who assisted the SS in burning the bodies of dead prisoners and ushering other prisoners into the gas chambers but were taken out and shot on a fixed basis for knowing too much. (However, not allSonderkommandocollaborated willingly.)
Non-Jewish traitors, however, were mostly exempt from this treatment. Even Slavs, whom the Nazis considered the next worst scum after the Jews.
In World War II during the Allied invasion of Vichy-held North Africa, French soldiers cut vital communication lines so orders to fire on the invading American forces couldn't go through. The pro-Nazi Vichy government (which was allowed to continue in power in North Africa for political reasons) later sentenced those soldiers for treason, and General Patton refused to exert pressure to get them released because to him a traitor was a traitor, no matter what the cause.
Then vastly averted when pretty much every other other Western Allied leader of note in the European War pretty much threatened the Vichy admin with utter annihilation if they went through with it on the basis that the French soldiers were not traitors to the (supposedly illegitimate) Vichy government (which was itself a traitorous regime of collaborators to the Nazis) but loyal to the Free French. Pretty much all of them were quietly transferred over to De Gaulle in order to avoid the resulting stink.
American voters seem to feel this way in regards to people who switch parties to win elections. Senator Arlen Specter lost his primary after switching to a Democrat after over 30 years as a Republican Senator. In Alabama, Representative Parker Griffith switched to the Republicans barely a year after being elected as a Democrat, and was hammered in a huge defeat in the Republican primary. On the other hand, Richard Shelby (also from Alabama) jumped ship to the Republicans in 1994 (the day after the party's midterm landslide!) and has been easily reelected in every election since.
Subverted with the Dixiecrats. The South used to be solidly Democratic, because the Republicans were the party of Lincoln, who had conquered the South and taken away their slaves. Likewise, many blacks voted for the Republican party, because their former slave masters were for the Democrats. The Democrats, however, were a highly populist party, and over time both parties became increasingly progressive, a trend which continued into the early 20th century. When FDR implemented many policies in the 1930s which helped blacks, many blacks defected to the Democratic party, and the Democratic party became progressively less racist. Eventually, the Democrats embraced desegregation and equal rights for people of all races. This enraged the southern Democrats, who were conservative and deeply racist, who briefly formed the so-called Dixiecrats before defecting en-masse to the Republican party over the course of several decades, a process accelerated by Richard Nixon taking advantage of them via the so-called Southern Strategy - appealing to Southern racists in order to get elected in 1968, but without any real intention of getting rid of the rights gained by blacks. It worked, but in the process the Republican party ended up picking up virtually all of the conservatives in the United States, concentrating them all into the same party. Once Nixon fell, they gained control of the Republican party and massively changed its agenda, leading to the so-called Moral Majority and Ronald Reagan. This was one of the most important shifts in American political history, and redefined the Republican party into what it is today.
A ridiculous example with Louisiana Governor Buddy Roemer, who switched to the Republican Party late in his term and was rewarded in the following election by coming in third to a former Grand Wizard of the Ku Klux Klan.
King Henry I of England had a man pushed off the tower of Rouen Castle for breaking an oath with Henry's enemy (and brother), Robert.
In the commercial/industrial aspect, there's this story about a woman who worked at and sought to sell Coca-Cola trade secrets... who was busted by Pepsi Co. She was sentenced to 8 years in jail.
Non-lethal variant; one day when Theodore Roosevelt was working as a rancher in the badlands, he caught one of his men trying to brand another rancher's cow with his, Roosevelt's, brand. Roosevelt promptly fired him, saying "if you steal for me, you will steal from me."
Another common not-so-fatal variant is often discussed in advice columns when mistresses write in seeking advice on the adulterous men they're trying to persuade to get a divorce. As the columnist almost invariably points out, if a man was willing to dump his wife for you because he got tired of her, how long do you really think you have before he'll get tired of you and start cheating on you too? The best bet for a mistress, therefore, is to "beat the rush" and dump her adulterous boyfriend preemptively.
The Battle of Bosworth Field in 1485 was decided by Sir William Stanley's defection from Richard III's side to that of Henry Tudor. Henry, later king Henry VII, had Stanley later beheaded for high treason.
Yet another non-lethal example would be the aftermath of Marshal Marmont's betrayal during the 1814 Campaign of France. Talleyrand had convinced him to abandon Napoleon at a crucial moment, accelerating his defeat, and then was the first to paint him as a dirty traitor, to the point that the word "ragusade"note Marmont had been made Duke of Raguse by Napoleon entered the French language as a synonym for "betrayal". He did receive some honors from Louis XVIII during the Restauration, but he was hated by both Napoleon's partisans and the Royalists and spent thirty-seven years - almost half of his life - as a complete outcast, despised and shunned by almost everyone.
A spectacularly damaging political example from Canada came in the form of Alberta politician Danielle Smith. Head of the far-right Wildrose Party, which had spun off from the governing centre-right Progressive Conservative Party over concerns it had shifted too far away from traditional conservative ideals, Smith was considered one of the rising stars of conservatism in Canada and continually held the government's feet to the fire over real and alleged misdeeds. However, when the Progressive Conservatives ditched their unpopular centrist leader, Allison Redford, in favour of the more right-wing Jim Prentice, Smith and he devised a deal that saw Smith, along with over half of the sitting Wildrose MLAs, cross the floor to rejoin the PCs in what was easily the largest party-to-party defection in the history of the province (and one of the largest in the country). Smith and Prentice had hoped this would spark the re-unification of the PCs and the Wildrose into a single party and there were rumours that Smith had been promised a cabinet post for the defection, but those were never confirmed because voter reaction to the defection was overwhelmingly negative and the punishment came swift and furious. Every single one of the floor-crossers that ran in the next election was soundly defeated; Smith didn't even make it that far, as her new party's voters rejected her in a nomination battle, effectively ending her once-promising political career. And, as the icing on the cake, both the PCs and the Wildrose lost the election to the left-wing NDP, ending almost five consecutive decades of Progressive Conservative rule and terminating one of the longest-standing political dynasties on the continent.