A subversion of We Can Rule Together. The hero joins the villains, but then one side betrays the other because they had no intention of sharing power in the first place. It tends to manifest in one of two ways.
Hero betrays Villain
The villain tells the "hero" (think Villain Protagonist, Fallen Hero, Tragic Hero, Byronic Hero, certain types of Anti-Hero, or Sociopathic Hero) that We Can Rule Together, and why would he refuse? He has built up lots of bad karma and kicked lots of dogs by now. The former hero likes the idea... except for that little "sharing" detail. As he points out, there is absolutely no reason not to just kill the villain and take the Cosmic Keystone for himself, ruling solo.
Or if that is too long, Villain offers a chance to rule together, "hero" kills villain, takes MacGuffin and uses it to dominate the world himself. Word of advice to villains: if you're dealing with a person who was willing to betray their true friends and comrades for a taste of power, don't be surprised that they'll be willing do the same to you. This is typically a Start of Darkness.
Villain betrays Hero
Some especially cunning Villains give the We Can Rule Together speech, allow the hero to fall, and then reveal that...gasp they were lying about sharing! Who would have thought? Alternatively, the offered Face-Heel Turn was just a test by the villain to test the hero's trustworthiness. Then after joining the bad guys, the former hero gets Rewarded as a Traitor Deserves because the villain has a code of honor or is Dangerously Genre Savvy. After all, why would the villain trust someone who betrayed his own cause?
After he discovers the conspiracy in They Live!, Nada is offered at least twice to join the aliens. However, both times they either don't mean it, or it's not really plausible. The two aliens disguised as cops are only saying it to get Nada to a quiet place where they can kill him, and Holly offering it at the end would never work out, since at this point Nada had already killed dozens of aliens and would obviously be killed in retaliation.
Quite common in Star Wars. In fact, this is more or less the modus operandi for Sith in general in the series, given that betraying and killing their masters is built into their dogma. The Rule of Two allows for a lot of Dark Jedi, even Dark Jedi with a bit of Sith training, but only two true Sith, and of those two, if the apprentice doesn't kill the master, the master will try and kill the apprentice. This is assumed from the start of pretty much every apprenticeship - the student will hold off on killing the master until enough has been taught or the master looks too weak, and the master will refrain from killing the apprentice until the apprentice becomes worryingly powerful. Or until they've done all the errands the master wants. Or until a more promising apprentice has been found and taken well into hand.
In the novelization of Revenge of the Sith, Anakin starts plotting to overthrow Palpatine the moment he turns to the dark side. However, he doesn't get a chance to put his plans into action.
Star Wars: Legacy has the Sith abandon the Rule of Two for the Rule of One - that 'one' being the Sith as a whole, meaning that there are a lot of them and they actually can work together, if uneasily.
In the Darth Bane books, Bane's problem was that his apprentice Zannah seemed willing to wait for him to die of old age so she could claim his rank without fighting. If they did not fight to the death, how would they know who had truly been the strongest? Bane acknowledges that waiting for an enemy to grow weak is a valid and normally wise strategy, but this was a special case because it was essential for Zannah to be stronger than him once she became the Sith Lord, otherwise the Sith Order would stagnate.
In Fate of the Jedi, and its Lost Tribe of the Sithtie-in, the aforementioned tribe seems to avert this. In the series proper, when their High Lord Taalon gets cut open from behind by Vestara, she has to get out of Dodge, to avoid being killed in revenge... Also, earlier, a Sith ship's captain and entire crew just let themselves get executed as a scapegoat, 'for the good of the tribe'.
Inverted in the Fate/Zero light novels; Rider was asking Gilgamesh to rule the world together with him.
The Stainless Steel Rat is seeking to depose a planetary dictator by fair means or foul in The Stainless Steel Rat for President. At one stage the dictator meets with Jim DiGriz in private and suggests We Can Rule Together — he'll run the government, DiGriz will run the opposition, and they'll quietly eliminate anyone who's a real threat. DiGriz refuses outright because he believes in democracy and thinks the dictator is a total scumbag. The dictator rejects this as all politicians are out for themselves. Fine, says DiGriz, and goes on a spiel about how he wants all the goodies for himself, "All the power, the money, the women" causing the dictator to shed Manly Tears. "You remind me of myself when I was young." The truth is that DiGriz is doing all this for fun, so he fakes his death at the end to get out of running the planet.
Star Wars videogames (see also Star Wars under Literature):
Kyle Katarn in the dark side ending for Jedi Knight, as mentioned above.
Likewise in Jedi Academy's dark side ending, Jaden Korr will kill Tavion and take the Scepter of Ragnos for him/herself.
The Force Unleashed has one of the surviving Jedi tell Starkiller, "The Sith always betray one another. But I'm sure you'll learn that soon enough." And indeed, not long after that Vader turns on his secret apprentice. Vader's other secret apprentice, from a manga storyline, was killed when the Emperor found out about him. Compared to Starkiller, though, little Tao comes off as something of a Morality Pet which Vader was not allowed to keep.
In the Revenge of the Sith videogame, one of the the endings has Anakin/Vader killing Obi-Wan on Mustafar, then subsequently killing Palpatine to take control of the Empire.
In Knights of the Old Republic, if you follow the dark side ending then Bastila, who's turned to the dark side and become Malak's apprentice, will decide to betray Malak and join you to overthrow him.
A cutscene in Samurai Warriors 2 (and Empires) shows Akechi thinking exactly this as he initializes the Incident at Honnoj, turning on his former master, Oda Nobunaga. His expanded reasoning is that he initially agreed to follow Nobunaga because he believed him able to put an end to these war-torn times... but then, watching the damage caused by Nobunaga's armies, he changed his mind. "Fuck that," he said "I'll just knock him off and become a lord myself. Then I'll bring peace to these troubled times." And so he did. 'cept for the 'bring peace to these troubled times' bit, maybe.
Amelyssan does this in Baldur's GateThrone of Bhaal. "Remain dust, my dead god."
In the bad ending of Darkwatch, after defeating Cassidy and helping Tala secure ultimate power, Jericho Cross rips Tala's jugular open and goes on to presumably take over the Wild West.
In The Suffering : Ties That Bind, Blackmore encourages Torque to kill Jordan, despite the fact that she's his most powerful ally and the two have a We Can Rule Together moment in the game's evil ending if you ignore him and spare her. Then again, this may simply be because Blackmore has zero impulse control.
The Neverhood game allows to choose this trope as one of its endings. Either you kick out the villain and take the crown for yourself, or rescue the legitimate king.
In Ultima Underworld II, Mors Gotha presents the We Can Rule Together offer to the Avatar. However, it's merely a ruse to get you to hand over a weapon; if you accept, she tells you that power was never meant to be shared, and proceeds to attack you.
Inverted and subverted in Dawn of War where Sindri is almost backstabbed by Brother-Librarian Isador for this reason but is quite ready for the attempt and betrays Isador first.
One of the possible ending choices in Alpha Protocol, depending on choices you make in the game. Depending on the contacts you made throughout you have varying levels of success with the aftermath - up to and including changing the entire course of global politics.
The first Streets of Rage has Mr. X posit the usual "join me and we can rule this town" spiel. Accepting it only serves to toss you back two stages. Two-player, however, works differently; both players can answer the question. If both answers are the same, it's akin to the single-player answers; if both players answer differently, however, a battle between the two players initiates. If the player who agreed to join Mr. X wins, the question is posited again; this time, you can say "no" and invoke this trope. The final boss with Mr. X begins as usual, but winning gives an exclusive "bad ending" showing you in charge of the X Syndicate.
In the original Ogre Battle, if you win with a low reputation, you take the throne of Zenobia by force. This is generally a Downer Ending, though one such ending is potentially less of a downer from your perspective. If you have a low alignment but a good Charisma, you build a Black Empire using dark magic.
You can pull this at the end of Singularity, where you can just vent both Barisov and Demichev rather than go with either of their options. This leads to the USSR collapsing, Katorga-12 blowing up, E99 mutants invading East Russia and you becoming supreme dictator of the United States of America.
One wonders why anyone would go along with Demichev's option, since he's basically asking you to hand the TMD over to him; he doesn't even have anything to offer you.
Demichev has a global empire in which he's offering you a high-level position if you play ball. The ending implies you eventually pull this on him anyway. Barisov is the one not offering you anything, he's just playing to your conscience.
In Vampire: The Masquerade - Bloodlines, you can certainly TRY this by killing LaCroix and opening the Ankharan Sarcophagus yourself. Only it doesn't work at all like you had hoped. There's a reason you're repeatedly cautioned in-game not to open it.
This is the fourth possible ending in Fallout: New Vegas and can be either good or bad depending upon your actions throughout the game. The more people you helped the smoother the transition of power becomes in the ending montage and the people are more likely to accept you as their new ruler.
In Dark Messiah Of Might And Magic, this is one of Sareth's options. Xana will actually advise you to do this instead of freeing the Demon Sovereign, since by that point she'd rather follow your lead than his. Or she just finds you more manipulable.
In World of Warcraft, this was the ultimate fate of Ner'Zhul, Arthas took up the throne of the Lich King and killed the last of his humanity. Ner'Zhul intended for the two to rule as two minds in one body, Arthas didn't feel like sharing the throne and killed him too, merging their personalities with Arthas' as the dominant one.
In Spider-Man: The Animated Series, Red Skull turns his son into a creature of immense power, Electro, and then proceeds to expand on how he is now going to conquer the world with Electro by his side. Electro asks why he should stand by anybody's side when he is the one with all the power, zaps his surprisingly unsavvy daddy and takes off to claim the world for himself. Except the Skull really isGenre Savvy, and is Crazy-Prepared enough to have a plan in the event of such a betrayal (to be fair, the plan was executed 50 years late- he was caught off-guard by the betrayal only because his son wasn't supposed to be the one who got the power in the first place. It was probably intended for some random mook.
His son also brings up the point that the Red Skull risked his life in the experiment. The Red Skull dismisses his son's complaints, saying that the risk was "minimal". This didn't exactly endear him to Electro.
Used in an episode of Disney's Gummi Bears, when Duke Igthorne makes the mistake of putting Toadwart, his subservient dwarf ogre sidekick, into a suit of magic armor...
BIONICLE: Web of Shadows: The Visorak King Sidorak has long offered to share his throne with his second-in-command Roodaka in the form of Unholy Matrimony. She bided her time to take him up on it, however, until she manipulated things in her favor; soon after which she said this and left him to die fighting a very strong, very angry enemy beast. (Unfortunately for her, the puppet general she'd set up flipped the script and gave Sidorak's armies an out, permitting them to reject her rule and abandon her for her treachery.)
Big Bad Makuta Teridax used his Brotherhood to complete his grand plan to seize power, committing Grand Theft Me on the resident Physical God and basically becoming one with the entire known world (revealed to be a truly gigantic robot), allowing him to control everything inside of it and then use it to conquer other planets. The moment this hijacking works out (and even a bit before that) he begins killing off his own species, as he believed the Makuta were the only ones with the knowledge and power required to execute "The Plan". While the original plan was for the rest of the Brotherhood to rule alongside their leader, only a few of them even thought of the possibility that they could be betrayed. Since their entire motivation for overthrowing the Great Spirit in the first place was pride and jealousy at being eternally below him on the ladder of power, of course Teridax didn't trust any one of them to not try and do it against him.
A variation in the Jonny Quest original series story "The Riddle of the Gold": the Maharaja tells Dr. Zinn's agent that soon they'll be the richest, most powerful men in the world. The agent poisons the Maharaja and declares, "There is room for only one richest, most powerful man in the world ... and he is Dr. Zinn." The agent is NOT Zinn in disguise. That's loyalty.