No, TV Tropes. You are the demons.
And Then John Was a Zombie is essentially a literal version of He Who Fights Monsters. It is a situation in which a character, frequently a protagonist, is turned into the very thing he or she has been fighting; a survivor of a Zombie Apocalypse may get bitten by a zombie, a demon hunter may be possessed, and so on.
A popular variation of this is for protagonists to have this thrust on them as an Emergency Transformation, and/or use (or attempt to use) Heroic Willpower to avoid the Face Heel Turn frequently associated with this trope and use the powers for the cause of good. Results may vary. Of course, it is also possible for this to happen to villainous characters, particularlynon-human villains, though it is certainly possible for this to happen to human villains, especially in stories where The Virus has strong presence, but is not the sole villainous force. Depending on the villain and the type of transformation in question, results can and often will vary even more than heroic characters suffering this.
If poorly foreshadowed, this can very easily be a Shocking Swerve. Opposite of Tomato in the Mirror, in which case the main character was the monster all along. Also contrast Can't Stay Normal. Compare Doomed Protagonist, in which this was set up as an eventuality (or even a possibility) earlier in the story, The Virus, The Assimilator, and for variations in which the transformation doesn't force or suggest a Face Heel Turn on the victim's part, Viral Transformation.
The Trope Namer comes from an infamous bad fanfiction called Doom Repercussions Of Evil. In the story the Doom Marine, named John, claims, "I must kill the demons.", only to have his radio contact respond with, "No John, you are the demons." For no adequately explained reason John then randomly turns into a Zombie.
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Anime and Manga
The Claymores who become Awakened Beings definitely qualify. Particularly galling in the case of Ophelia, as she really hates Awakened Beings, and became a Claymore to fight them. And Then Cassandra, Hysteria and Roxanne Became Awakened Beings after they were resurrected.
In Digimon Savers, the main human villain, Kurata, has a great fear of all Digimon and kills a bunch of them in the name of one-sided peace, but then uses their life force to power an even worse villain-type Digimon and become one with it to ultimately take over the world.
In Gensoumaden Saiyuki, Hakkai slays a thousand demons and his thousandth turns him into a demon, pointy ears and all.
In Kakurenbo, the main character wins the game of hide-and-seek by being the last child to be found... and then is given the honor of being the new "it", that is, he becomes a demon himself.
Kannazuki no Miko: And then Souma was a petrified Orochi. He saw it coming, though, and try to use the Orochi powers contaminating his body to make a Heroic Sacrifice. He recovers later.
Magical Girls in Puella Magi Madoka Magica are normal girls who Weasel Mascot Kyubey gives a Soul Gem that lets you use magic and transform into a sparkly fighter of justice in order to defeat monsters, all in exchange for a wish. What Kyubey doesn't mention when you make your contract is that your Soul Gem, which blackens as you use up your magic and requires regular cleaning with Grief Seeds, is literally your soul ripped from your body. And what happens when it turns completely black? Every magical girl who doesn't die in battle will eventually become a Witch, the monsters that the magical girls fight throughout the series, and their Soul Gems become Grief Seeds, thus perpetuating a vicious cycle. This is especially illustrated in Episode 8 when this happens to Sayaka.
Even before you become a Witch, there is still some opportunity for horror. Since your soul is outside your body, that essentially makes you a girly, sparkly Lich. The girls find this out the hard way when Sayaka's soul gem is thrown too far away from her body, causing her to fall over dead until it is returned to her. She then considers herself inhuman, quickly leading toward her inevitable transformation into a Witch.
In School Mermaid, the main character ends up being pulled down into the floor by the very mermaids she was hunting and becomes one herself. And it is strongly implied that her best friend will hunt her down and eat her later.
In Shiki, almost every character ends up as a vampire, including two of the three protagonists.
In Uzumaki, after the teacher smashes a bunch of human hybrid snail eggs in disgust, he returns a week later a snail man. With a bunch of snail eggs growing on his back.
In the end, Kirie and Shuichi become spirals.
In Shingeki No Kyojin, members of the 104th Trainee squad encounter an Abnormal Titan that only attacks other Titans. It's actually Eren, as he's able to create and "pilot" a Titan body. He's not the only one. Reiner, Bertholdt, and Annie all turn out to be Titan Shifters sent from somewhere outside the Walls, with the mission to infiltrate the military and exterminate "mankind within the Walls". Eren's transformation changes their mission, with them attempting to kidnap him. Reiner claims they no longer intend to carry out the extermination. Ymir is also a Titan Shifter, but comes from somewhere else, suggesting at least two separate groups of Titan Shifters outside the Walls. The secret experiments Eren's father was carrying out in his basement may be the reason behind his powers.
Card Games
Magic: The Gathering gives us Karn, a golem created to fight the Phyrexians. But his heart was a Phyrexian stone, normally used for insta-You Have Failed Me. This stone contained, like all artifacts of Phyrexian origin, the oil, which took hold on Mirrodin. Karn almost gave in to the Praetors' whispers.
The Innistrad Block has a fetish for these, and with good reason, as the story line has nearly all monstruousities coming from mankind. In particular, "church warrior becomes zombie" seems to be very popular.
Some Yu Gi Oh cards have images that tell stories. One is the Gagagigo series; it starts as the Gigobyte, who wants to fight monsters... and ends with Gogiga Gagagigo, whose soul has ceased to exist and his body only moves on a drive for power. Then there's Warrior Dai Grepher; the first card of his story shows him and other warriors and spellcasters fighting demonic creatures. Then the series goes on and he becomes Dark Lucius. In Sakuretsu Armor, Dark Lucius LV 6 can be seen striking down some of the people he fought alongside in that first card.
Gigobyte starts down the road to evil soulessness by modifying himself to get more power to defeat the Invader of Darkness. Since the story is spread over the flavor texts of many cards over a few different expansions, it doesn't come as too much of a surprise. The compiled story is here if you're interested.
Comic Books
The ending of 30 Days of Night involves the hero, Eben Oleson, turning himself into a vampire to save his friends. This counts because the vampires invaded to feed, not to create new vampires and the humans (until that point) weren't interested in becoming vampires.
In the fictional comic book Tales of the Black Freighter, a story within a story from Watchmen, the protagonist mistakenly bludgeons his wife almost to death while attempting to save her and his family from undead pirates. The pirates only want him though, and the story ends ambiguously with the protagonist either being killed by the pirates or becoming one. His last words are 'I was a horror; amongst horrors must I dwell'.
This beautiful piece is a rewritten version that both triples (or something) the length of the original story AND gives it a better feel overall. Still not as good as the original, though.
Zombies has multiple cases of Decoy Protagonist syndrome. But finally, Princess Peach and Sonic seem to be the Big Damn Heroes of the story until they both succumb to their infections and devour their allies viscously.
In Grandma Got Bitten By A Vampire (a fan sequel to the animated Grandma Got Run Over By A Reindeer). Daphne manages to cure her family of vampirism back to normal only to find that their dog, Doofus, was turned and he promptly attacks and feeds on her. What's more Grandma had made friends with the vampire who bit her and liked being undead (as she was in her twilight years anyway) and thus regained her vampirism. Re-vamping Jake (who was initially against being a vampire) as well. The three decided to keep the secret from the family for the time being and enjoy terrorizing Cityville at night.
In Respect: This happens in a symbolic sense to Yayoi, who makes a contract with Kyubey in order to stand up to the bullies that torment her. By the end, as Mami points out, she's become a villain, a "bully" herself, and being confronted with this sends her over the Despair Event Horizon. The trope then happens in a literal sense, as we see that Yayoi has transformed into Izabel, the Artist Witch from episode 10 of Madoka.
In the canon ending, the Elements of Harmony transform almost every changeling into a true pony permanently.
In the alternate ending, Twilight willingly becoming a changeling in order to take over the hive.
Film
At the end of the 30 Days of Night movie, the protagonist has to become a vampire in order to fight off the vampires that have been eating everyone. He dies very shortly thereafter due to watching the sunrise with his girlfriend (so he'll die and not eat anyone).
A homage to such endings occurs in the 1996 Tales from the Crypt movie, Bordello of Blood Rafe saves his love interest, Catherine, from Lilith. Only at the end to find out Lilith managed to bite her in the thigh, conveniently hiding the bite make from him. Catherine been undead since her rescue and proceeds to bite Rafe once she and him are alone.
At the end of Candyman the heroine becomes a Candywoman.
At the end of The Chronicles of Riddick, Riddick kills the Grand Marshall and becomes the new leader of the necromongers, which was foreshadowed very strongly early on, when Riddick is told that the Necromonger way was "You keep what you kill".
The ending of the 1970 film Count Yorga Vampire The last survivor Michael manages to go to his girlfriend, Donna, and kill Yorga. After chasing off Yorga's remaining brides (one of which who was friend of theres) Michael thinks its over. But just as he relaxes, Donna lunges at him, now a vampire herself, and kills him.
A reversal of the kind of ending mentioned above occurs in the 1971 sequel, The Return of Count Yorga, Hayes manages to get past the vampire brides and confront Yorga. After a fight, Yorga is killed by Cynthia, a girl he was trying to turn. Cynthia hugs Baldwin only to pull away and reveal he's just now turned into a vampire himself. Baldwin promptly goes to bite her and bring Cybthia into the undead ranks he was trying to prevent not moments before.
In the final scene of the movie Deadgirl we find out that the main character has become a zombierapist despite spending the entire movie trying to dissuade his friends from doing the exact same thing.
In the Doom movie, Reaper is dying and his sister subjects him to the chromosome which had turned everyone else into monsters. Luckily (and perhaps subverting this trope) he is among the small percent of humans who don't turn into monsters when exposed to it and instead become 'angels'.
Arguably played straight by the "protagonist" Sarge turning into a demon.
Sarge remains human for the entire movie and never shows any physical or mental signs of becoming a demon. Sarge, like Reaper, is a human who's heart is pure. Sarge's mission is to sterilize the infection, and he's simply doing his job.
Occurs in Dragonball Evolution when Goku turns into the Oozaru, said to be an unstoppable servant of Piccolo, after having spent the movie determined to defeat Oozaru. Only after killing Roshi does he get out of it. This is not the case in the original source material, where the Oozaru is not mentioned until Goku transforms for the first time.
In the end of Roman Polanski's 1967 film The Fearless Vampire Killers (and the subsequent 1997 musical adaptation, Tanz Der Vampire), the protagonists rescue the heroine from a pack of blood thirsty vampires, only to have her turn into one and attack the "hero" of the film at the very end.
Halloween 4 The Return Of Michael Myers had this at the end with the little girl holding Michael's butcher knife, implying that she has become the new killer.
The uber-creepy 1978 film of Invasion of the Body Snatchers ends with Veronica Cartwright's character approaching Donald Sutherland's character (he was the protagonist). As she calls out to him, he wheels around, points at her, and emits an inhuman wail (sort of the Pod People's call to arms).
Needy becomes part-demon after surviving Jennifer's attack in Jennifer's Body.
The Lair of the White Worm ends with Hugh Grant's character lunging for another member of the cast after having been turned into a vampire.
In the 1990 remake to Night of the Living Dead Ben is badly wounded by Cooper during the night and as a last resort locks himself in the basement against the zombie horde. When we last see him, the door seems to be holding, but Ben has no supplies and his wound is worsening. When Patricia returns the next day with a group of locals, they eventually manage to open the door to the basement, and encounter the now zombified Ben.
In Pumpkinhead, a man summons a spirit of vengeance (the titular Pumpkinhead) after his son is accidentally killed by reckless teenagers. Realizing how screwed up things have become, he tries to stop Pumpkinhead, to do so, he himself must die. At the end of the film the man has to become the next Pumpkinhead.
All the alternative endings of The Ruins show the Final Girl succumbing to the evil plant spores in some way, while it is suggested in the theatrical one.
David Cronenberg's Shivers...and then Dr. Saint-Luke was Infected. Ironically, David Cronenberg considers this a happy ending, as he's said the Infected are happy and free of their inhibitions, and at the end they're about to share their newfound freedom with the whole world.
Skyline has Jarrod's brain installed into an alien, only for his Heroic Willpower to grant him control over that alien instead of just becoming its CPU.
At the ending of The Thing (1982), it appears as though the Thing was killed in an explosion. However, Mac and Childs are still around, and it's not revealed whether or not they had been infected, leaving them to wait and see what happens. The most optimistic reading possible is that they both freeze to death.
In Timecrimes, Hector gets stabbed in the arm and chased through the forest by a man in a black coat with his head covered by bandages. Then he travels backwards in time an hour, cuts his head, and begins bandaging the cut. Realizing that he's in a Stable Time Loop, Hector completely wraps his head in bandages, then takes a pair of scissors and attacks his past self, deliberately causing the series of events that led to his climbing into the time machine an hour ago.
Near the end of Undead or Alive Elmer makes the mistake of punching an infectious zombie in the mouth, and almost immediately realizes that he is beginning to turn. While he uses the last few moments before the hunger overwhelms him to attempt to make a Heroic Sacrifice, he succumbs in the end, infecting one companion and convincing him to help devour another.
At the end of Vampire Assassin, the protagonist becomes a vampire.
In Van Helsing, the Live Action movie, Van Helsing becomes a werewolf to battle Dracula because apparently vampires are weak against werewolves.
A variant occurs in Underworld. Decoy Big Bad Lucien is attempting to turn Michael Corvin into a Vampire/Werewolf hybrid as a means of ending the war between the species while Selene investigates the Lycanths' interest in him and attempts to protect him. He nonetheless succeeds in turning Michael into a werewolf as the first step in the process, and when Selene's Vampire Dad Viktor is awakened and learns of this, he is determined to kill Michael and prevent it. Selene instead completes the process herself to stop Viktor and Michael becomes a hybrid.
Carved has the main character being possessed and becoming the new Kuchisake-onna/Slit-Mouthed Woman.
In The Midnight Meat Train, Leon becomes the new Butcher at the end after he defeats his predecessor.
Subverted in Eden Log. Several times the man is shown to lose his mental control and starts behaving like one of the feral mutants, but he never fully undergoes the transition.
Ripley in Alien: Resurrection. Due to genetic mixing Ripley 8 has inherited a lot of Alien characteristics, and sympathizes with them to a certain extent. This version of Ripley is technically a different character, but the irony has to be appreciated of Ripley becoming so much like the Aliens after she fought them for so long. When she's being brought to the Queen by one of the Aliens after she's captured, she actually hugs the thing as if she's finally come home.
An alternate ending in From Dusk till Dawn 3: The Hangman's Daughter show one of the protagonist, Ambrose Bierce, telling his tale to a bar patron. When the man doesn't believe his story, Ambrose offers proof...by promptly pulling the man's heart out. The final shot is his eyes changing to a gold color, him gaining fangs and growling inhumanly before he eats the heart.
In P C Hodgell's Chronicles of the Kencyrath series, a singer named Ashe is bitten by haunts (ghouls with an infectious bite) during a battle early in the second book but hides her wounds. By the time of the final battle in the book, she is technically dead (but hiding it well and fully in control of herself), and plays a key part in the main protagonist's (Jaime's) survival: Ashe guards Jaime's back while taking wounds that would have killed a living Kencyr many times over. Ashe's motives are unclear but seemed to include dispassionate curiosity.
Calling All Creeps. Some reptilian monsters that can turn into human form come to think that the protagonist, a boy bullied by most of his school, is one of them. They have plotted how to transform everyone in school, and then on town, country and world, in Creeps like them and the hero is trying to stop them from feeding everyone the transforming goo. In the last moment, when he is mocked one more time while trying to stop everyone from eating goo-filled muffins, he is told that he will be the ruler of all Creeps and no longer a target for bullies. The protagonist does a quick Face Heel Turn, treats everyone to eat the muffins, and eats one himself, becoming the real Creep leader.
In Hater by David Moody, the main character becomes a Hater about three-quarters of the way into the book, after a prolonged struggle to defend himself and his family from them.
In John Dies at the End, it is revealed in the penultimate chapters of the book that somewhere along the course of the story, Dave has been replaced with a "monster" version of himself, becoming the very doppelganger that part of the story's arc has revolved around. Considering the character John neither dies nor suffers Dave's fate of becoming that which they fought, this could be a subversion. Especially likely considering the writer is Cracked Editor David Wong.
In Necroscope, vampirisation happens to the good guys every now and again. Eventually, Harry Keogh is subject to this as well.
In the Newsflesh book Feed, Georgia, the narrator, gets shot up with Kellis-Amberlee virus and turns into a zombie and has to be killed by her brother.
Shinigami ::God of Death:: by Django Wexler has the younger of two sisters bring her mother and her older sister back to life, using the same method the Lightbringer did. Unfortunately, this causes her to be imprisoned as he was.
Waves Put Out The Wind by the brothers Strugazkiys has a commission that ensures that no Sufficiently Advanced Aliens can improve human history unnoticed. One of employees gets some hidden superpowers activated while investigating a superhuman organization. He remains well-intentioned but becomes too inhuman to not gradually his contact with humanity. All "ludens" (anagram of inhuman in Russian and a reference to "Homo Ludens" - human that plays) have this fate.
Galaxy of Fear: City of the Dead has Zak Arranda caught up in the experiments of Doctor Evazam, who had noticed that recently dead bodies on Necropolis sometimes became mindless zombies after being exposed to boneworms. Evazam found out how to replicate their results, that these zombies followed his orders, and altered results so that the undead might retain some mind and memories. He used mindless zombies to kidnap one of Zak's friends, killed the boy and used that version on him, then had the zombie child help him kidnap Zak, doused him with the formula, and had him Buried Alive. Later Zak was dug up and found that his friend had enough mind left to help when Zak pleaded with him, but later returned to being a normal corpse along with the other undead after being exposed to an anti-formula. Zak himself was not exposed, and although he resisted any suggestion that he had died during his ordeal and was now a zombie, he was shown spasming uncontrollably as they sometimes did.
Live Action TV
Twin Peaks ends on a Cliff Hanger as Dale Cooper becomes possessed by the very entity he's been fighting throughout the course of the series. Could've been resolved in a third season but... Executive Meddling.
Are You Afraid of the Dark?: In "The Tale of the Chameleons", Janice gets turned into a chameleon and drowned in a well, and her Evil Twin plans to do the same to her friend and the rest of her family.
The Doctor Who episode "Survival" has this as the major plotpoint.
Also in "Asylum of the Daleks" where Oswin was a Dalek the whole time. She was only dreaming she was a human.
Dollhouse Season 2. Paul Ballard spends season one and most of season two trying to break down the dollhouse and free the dolls. But he is then sent into a brain-scarred coma by Alpha and is only saved when Topher rewires his brain with the active protocol and rewrites some part of his brain to work in place of the scarred tissue, effectively turning him into a doll.
Many of the characters in the live action adaption of Goosebumps, most follow the books to the letter. But a noticeable deviation is Vampire's Breath, in the book the whole adventure was played straight in which the protagonists spend the whole episode trying to avoid getting bitten and eventually succeed (though one of them takes a substance meant for werewolves). In the TV version however, the two find out that their whole family are vampires (the one chasing them was actually a relative) and that the siblings have now come of age where they're fangs have grown in.
In Kamen Rider Decade, this happens to AU!Hibiki. In fact, this trope is a common theme throughout the franchise, although the difference is that they were the "zombies" in the first place or take the source of their power from the very monsters they fight.
Also happens in Kamen Rider OOO: Eiji is slowly becoming a Greeed due to the purple Core Medals entering his body. He eventually transforms into a second Dinosaur Greeed after Dr. Maki inserts two more purple Medals into him.
How much does Joe of The Lost Room love his daughter? Well, after spending all the series trying to find the Object to rescue her from a Negative Space Wedgie, he becomes an Object to save her. That's right, he gives up being a human and becomes a sapient, mobile, indestructible and unaging thing. This is presented as a good thing however, despite the former Object/person/Occupant pretty much hating his existence.
The end of Supernatural season three seems to apply to Dean once he finds out that demons are ex-humans. He also gets tortured enough in hell so that he finally breaks and starts torturing others and enjoying it because it isn't him..
By the end of season four, this trope applies to Sam. No, Sam, you are the demons.
Used again with Gordon, who started out as a vampire hunter and is eventually captured and turned by a group of them.
Tara from True Blood. She hates vampires after being kidnapped and raped by one in the third season, and spends the fourth season helping the Big Bad try to wipe them out. She is killed when she takes a bullet to the head for Sookie, and the fifth season begins with Lafayette and Sookie begging Pam to turn her. It works, and Tara takes the news about as well as you would expect.
In the 1986 Twilight Zone episode "Monsters!", the presence of a (kind-natured) vampire in their neighborhood causes every normal person in suburbia to temporarily mutate into a pack of gruesome monsters that kill the vampire - afterwards, they're horrified at the murder, never realizing that they were the monsters.
The Vampire Diaries does this multiple times. First with Caroline's dad who choses not to complete the transformations and later with Alaric who is turned into an Original Vampire just to become the world's greatest vampire hunter.
Finally, the Season 3 finale brings the comparisons between Elena and Katherine to their logical conclusion and ends with Elena turning into a vampire.
Mythology
Used in this Nigerian folktale.
A hunter goes into the bush. He finds an old human skull. The hunter says: “What brought you here?” The skull answers: “Talking brought me here.” The hunter runs off. He runs to the king. He tells the king: “I found a dry human skull in the bush. It asks you how its father and mother are.”
The king says: “Never since my mother bore me have I heard that a dead skull can speak.” The king summons the Alkali, the Saba, and the Degi and asks them if they have ever heard the like. None of the wise men has heard the like, and they decide to send guards out with the hunter into the bush to find out if his story is true and, if so, to learn the reason for it. The guards accompany the hunter into the bush with the order to kill him on the spot should he have lied.
The guards and the hunter come to the skull. The hunter addresses the skull: “Skull, speak.” The skull is silent. The hunter asks as before: “What brought you here?” The skull does not answer. The whole day long the hunter begs the skull to speak, but it does not answer. In the evening the guards tell the hunter to make the skull speak, and when he cannot, the guards kill the hunter in accordance with the king’s command.
When the guards are gone, the skull opens its jaws and asks the dead hunter’s head: “What brought you here?” The dead hunter’s head replies: “Talking brought me here!”
According to Japanese mythology, killing many youkai (a thousand, according to some versions) results in their slayer being transformed into a youkai himself. Then again, anyone capable of killing 1000 demons is powerful enough to be considered a demon himself by most people, so this could just be a Lampshade Hanging on that fact.
Music
Black Sabbath's "Iron Man." Through a Stable Time Loop, a man travels to the future and witnesses The End of the World as We Know It at the hands of the titular villain, then travels back in time to warn people about the coming threat, but in the process is transformed into Iron Man himself.
Tabletop Games
This is a quite common fate for any investigator in Call Of Cthulhu, ranking perhaps just below "killed by horrible monster existing in fourteen dimensions at once," "lab rat or soul-in-a-jar for horrible monster," "spends remainder of his or her life eating cockroaches in a padded cell," and "gets brain stolen by horrible monster."
The end result in Changeling The Lost for changelings whose Clarity gets too low and their Wyrd too high. They leave for Arcadia and become exactly the same as the monster who kidnapped and abused them in the first place.
Warhammer and Warhammer 40000 use this trope heavily. In 40k, it is Turned Up to Eleven, where not only do you turn into a demon, but you also have a good chance of winding up causing the deaths of billions. The pen and paper RPGS Dark Heresy and Rogue Trader feature extensive rules on corruption by the Ruinious Powers just to fit this trope.
Black Crusade takes this trope and runs with it. With trying to become a high-ranking demon a being a valid goal for the player.
In Dungeons & Dragons several undead have the create spawn ability, turning a victim into an undead (often the same kind as the killer) under their control.
Theater
Alfred, the assistant vampire hunter in Tanz Der Vampire, becomes an undead bloodsucker himself at the end of the show. Worse, the vampire that turns him is none other than Sarah, the girl he loves and was trying to save. However, in the Musical this isn't depicted as especially bad, as due to sympathy-songs etc. the spectators are expected to have sympathy with the vampires and the only one really wanting to kill vampires was the professor.
At the end of Diablo, the hero defeats the titular boss and jams its soulstone in his/her own forehead to contain it. This results in the hero becoming the new Big Bad in Diablo 2. This was later retconned in Diablo 2 by saying that said hero was more or less mindraped into doing so.
It's uncertain how long Tal Rasha held out containing Baal this way, but considering the Diablo 2 expansion, it's probably safe to say that anyone who fights demons succumbs to this in some degree.
And again in Diablo III. Leah, niece of Deckard Cain starts off as a genuinely good character. Then she finds out that her Mom is an evil witch, and her dad is the wanderer from Diablo II. She becomes corrupted by the soulstone after capturing a few more demons in to it (thus becoming an even more powerful version of) Diablo herself.
In Starcraft, Kerrigan fights the Zerg during the Terran campaign but becomes one during the Zerg campaign.Shegets better... and then she re-infests herself for the second time.
Similarly, Arthas and Illidan in Warcraft III. Note that in the cases of Kerrigan and Arthas, the trope is in that they become leaders of the factions that they opposed, not those that they first joined.
Sylvannas fights Arthas' undead right up to the point where she was killed and resurrected as a banshee.
The worse endings of BioShock have you becoming just as much of a monster as Andrew Ryan and Frank Fontaine.
Also done to a degree earlier. In order to complete the game it's necessary to be converted into a Big Daddy, the most dangerous and iconic enemies of Rapture.
BioShock Infinite also gets in on the action, revealing that Booker and Comstock are the same person, only Booker in this universe did not run away from his baptism, and the Booker you play sold his infant daughter, Anna, to "wipe away the debt".
This is the bad ending of The Breach. After the most recent update, it's the only ending.
In Castlevania 64'sDowner Ending, you get this on two levels; a side character, Vincent the vampire killer, becomes a vampire. Also, Carrie agrees to marry Malus (Dracula), to which he ominously notes that they have "a binding contract".
If you unlock the true ending path of Aria of Sorrow, then upon defeating Graham Jones, who has declared himself Dracula's reincarnation, Soma himself is revealed to actually be the reincarnation of Dracula. And if you get the bad ending, by losing to the True Final Boss, then he quite literally becomes the "second coming" of Dracula, in mind as well as ability.
In 6 Days a Sacrifice, the main character, Theo DaCabe, is turned into the mindless servant 'The New Prince' by Chzo after the old servant Cabadath tries to betray him, because Cabadath didn't want to be replaced.
The ending of the original Condemned seems to imply that main character Ethan Thomas has become one of the demonic freaks causing all the chaos in the game. The sequel josses this, as Ethans winds up as a burned out alcoholic bum, and the demonic freaks are really just an ancient mystical cult.
Even then, it still could be a case of this trope; in both games, the primary enemies are pyschopathically violent homeless people, driven mad by the real villains. In Bloodshot, he's suffered substantial Sanity Slippage and starts off pretty much only slightly less unhinged than the psychopaths he fights.
Dead Space Extraction: In the first chapter, you play as one of the soon-to-be necromorphs and unknowingly kill all your comrades.
Then at the end of the game one of the main characters turns into a necromorph too. Though considering the later DLC for part 2. It was revealed as being a dream.
Disgaea 2'sworst ending features this. In spades. If Adell has met the required conditions, Rozalin, the real Overlord Zenon, will inform you that you too are her enemy. If you are victorious, Zenon's evil will consume Adell's soul. His first act as the new God Of All Overlords? To devour his younger brother and sister, bones and all, ALIVE. Cue crunching sounds.
In Dungeons of Dredmor, the first 3 levels of Demonologist give you resistance to Righteous damage, and useful powers when fighting demons. Any further than that, and you lose all that resistance in exchange for more versatile moves. What's the name of the penultimate level, giving you a chance to transform in to a literal combat monster? 'No, you are the demons'.
The good ending of Eversion turns your character into an Eldritch Abomination just like the princess you're out to save. The bad ending has you resisting this and getting eaten by the princess.
Eternal Darkness: Anthony's chapter casts the hero as a Frankish messanger who intercepts a message intended to curse Charlemagne by exhausting the curse on himself first; his chapter is devoted to racing to defend Charlemagne while slowing succumbing to the corrosive curse though his efforts are futile. In Paul Luther's chapter later on, Anthony is encountered again, tragically having been turned into a zombie of Augustus Pious's dark god.
The ending of Fable: The Lost Chapters gives you the option of putting on Jack's mask, becoming his new host.
In Gem Craft Chapter 0, the force you're trying to claim takes over, but it's a prequel and the truth was fairly obvious based on the events of the first game.
Happens in one of Kichikou Rance's endings, where Rance, depressed either for Sill's death or absence (if the player is taking too long in rescuing her) finally gives in to Satera's suggestions of becoming the Demon King, committing immediately after a Moral Event Horizoncruelly killing poor Miki, who, crushed by the betrayal, lets herself be hacked during hours, since she's that tough. Then he becomes a powerful Demon King, who crushes without trouble the human nations, with Satera at his side.
In the bad ending of the first Kid Icarus game, Pit is turned into a monster. Oddly, it's Palutena that does it to him. Maybe this is really supposed to be the joke ending?
In La Pucelle, if you defeat certain powerful enemies in the Dark World/Netherworld (which requires a lot of Level Grinding), a band of demons will appear and declare Prier their new ruler... which Prier rejects out of annoyance after being teased by her teammates, and the game continues anyway, ignoring this interruption. However, if she continues on past that, becoming even stronger, she triggers a Non Standard Game Over as the demons make her their new ruler for good and her friends abandon her. Prier being a demon queen is the canon ending, as she puts in several appearances in the Disgaea series as "Demon Overlord Prier," a powerful optional character, as a half-demon with wings and a horn.
In Legacy of Kain: Soul Reaver 2, it turns out that the Soul Reaver is in fact Raziel's soul, which was absorbed into the original blood stealing "Reaver" and driven insane from being trapped for hundreds of years. And that he is doomed to fulfill this fate over and over again at least until Kain creates a paradox and saves him... until Defiance when Raziel willingly becomes the Soul Reaver, purified of any corrupting influence. In true mind-breaking Legacy of Kain style, this is alsoTomato in the Mirror. There's a Tomato in the Mirrorbecause John became a zombie at the end.
In Oersted's chapter of Live A Live, he spends most of the chapter trying to vanquish the Demon King. Then, after everyone around him either dies, betrays him, or turns against him due to a really big misunderstanding, he decides to become the Demon King, thus setting the stage for the rest of the game.
The Hidden Object GameLove and Death: Bitten features Daemon and Victoria, the former a vampire and the latter a human trying to help him regains his humanity. At the end of the quest Their roles are reversed. Daemon is turned back into a human, but in trying to beat the vampire queen that turned him. Victoria is captured and bitten, becoming a vampire.
At the end of the second act of Marathon: RED (a Game Mod), the hero is mutated by The Virus, and his former allies turn against him for a stage, but he turns Phlebotinum Rebel.
Present in one ofMass Effect 3's endings. Shepard, the protagonist, takes over the Reapers, effectively becoming their unified intelligence. How s/he handles it depends on the player's Karma Meter-if you play as a Paragon, s/he focuses on protecting the galaxy, but if you're Renegade, s/he places more emphasis on eliminating potential troublemakers.
Also something that happens to a lot of people trying to research Reaper tech... like the entirety of Cerberus.
Happens in Metal Slug 2/X and 3, with MS2/X setting the players against mummies, and MS3 being the literal trope as zombie enemies can make you one of their own (though not without benefits). The clones of your abducted comrade? They join in on the fun, too!
In the escape-the-room game Poco Escape, you wake up locked in a room that's sparse except for a few pictures of adorable little bears, which are naturally slightly creepy considering the circumstances. The ending heavily implies that you are actually the girl's teddy bear.
In Portal 2, you spend the first half of the game with a robotic sphere named Wheatley trying to defeat GLaDOS. Guess what happens to Wheatley?
In Prince Of Persia Warrior Within, when the Prince puts on the mask, he finds himself transformed into the Sand Wraith. This makes him not only a monster but a doomed monster, as he saw the Dahaka kill the Sand Wraith in the past. Fortunately, he is ultimately able to Screw Destiny and return to his normal self.
Happens in Prototype to several characters. Alex: "I am the virus!"
Possible example: In Quake IV, at a particular point that marks roughly the midpoint of the game, the protagonist is converted into a Strogg in all but allegiance. Were it not for a very well timed raid by the good guys, our hero would have crossed the point of no return, as he was almost the entire way through the conversion process by that point. He then became a Phlebotinum Rebel. Wanna see?
The Bydo love pulling this on humanity's heroes in the R-Type series.
It first happens in R-Type Delta, if the player is piloting the R-13 Cerebrus—that particular fighter proves incapable of escaping the final level, and the Bydo exact revenge by converting the ship and its pilot into a tree-like Bydo.
Then there's R-Type FINAL, in which one of the Multiple Endings leads to the player getting converted into a hybrid Bydo/human/R9-Series...thing...and is sent back to the first level of the game, fighting both the Bydo as well as its former comrades. (It's worth noting that at the start of the game, you can see your future Bydo self fly by at the very start of the level!)
R-Type Command also pulls this on an entire human fleet at the end of the Human campaign—in fact, the player's character in the subsequent Bydo campaign is the former human commander. In both the case of FINAL and Command, the player's characters have no idea that they're no longer human and can't understand why Earth's forces are out to get them.
At the end of Red Dead Redemption's Undead Nightmare story, the main character returns from the dead as one of those zombies he has spend the whole game fighting (thankfully, due to some special circumstances, he turns out to be a sentient one). And the character's name? John Marston. The Achievement for completing the game directly quotes the Trope Namer.
In the end of Resistance 2, Nathan Hale fully mutates into a chimera.
The ending of the incredibly obscure computer game THE SCREAMER. (Yes, it really is in all caps like that.)
Shadow Of The Colossus has Wander gradually becoming less human, and ends with Wander becoming a new version of Dormin's body (which is what the Colossi originally are) himself. It fails, and he reverts to human form, but as a newborn with demon characteristics. It's heavily implied that he becomes the ancestor of the horned boys in ICO this way.
In the Possessed ending of Silent Hill 3Heather becomes demon-possessed, and kills Douglas.
Silent Hill 3 uses this trope earlier, when Vincent tells Heather she has beeen killing people, not monsters, making her the possessed demon. This being Silent Hill, whether or not he is telling the truth is deliberately left vague.
In Silent Hill Downpour, Murphy becomes the Bogeyman for the final fight. It's not permanent, though, assuming you choose to spare Anne.
Played for laughs in the Hidden Object GameSinister City. In the end, Count Orlok becomes the star of a movie in which he plays a vampire. Scenes from the film shoot are shown during the game's credits. John, the protagonist, plays a victim of the vampire in one shot with Orlok ready to bite him. Orlok bites him for real and the director calls for an ambulance. The last shot is John as a vampire.
In Streets of Rage, the player is one of three ex-police officers finding themselves duty-bound to stop the assorted crimes of Mr. X, a powerful crime kingpin. In the final level, Mr. X offers to make you and your co-op partner into his right-hand men. If one player accepts and one refuses, Mr. X sits back and watches while you and your former partner solve the disagreement the only way they can. If the "heroic" player loses the fight, the remaining player goes on to fight Mr. X, and if successful...No, Axel, You Are The Crime Lord.◊
Possible again in the fan remake for any character, including Mr. Freakin' X. Paradox anyone?
In the end of Super Paper Mario, Luigi ends up as part of Dimentio's plan and pilots a giant robot before being cured by Mario. It was implied something might happen to Luigi a few minutes before that, though.
Happens in the end of Throne Of Darkness, to the Magnificent Seven Samurai. After defeating the Dark Warlord, their feudal lord reveals that he only wanted you to retrieve the immortality potion. He then drinks the potion, transforms into the next dark warlord, kills his own servants (the player's part) and brings them back as zombies.
The backstory of Byakuren Hijiri in Touhou 12 is this played sympathetically; originally a youkai-hunting priestess, Byakuren came to appreciate her youkai allies more than her human ones, and that combined with the fear of death drove her to transform herself into a youkai. Sanae recieves a warning that she who hunts youkai may become one, as well. In her profile, it's mentioned that Alice was a human who became a youkai to pursue power. In supplementary materials, Marisa is confirmed to experiment with methods of gaining immortality, but in one game she refuses to eat an immortal person's liver to get it. Reimu also has that option, and doesn't balk at the method so much as the idea of becoming anything remotely inhuman.
Marisa is a curious exception to magicians in general; it's stated All There in the Manual that becoming a Youkai Magician is a natural progression of studying magic. Marisa has made neutral comments towards immortality; and Alternative Character Interpretation goes both ways towards whether she's heading towards becoming youkai; or if she'd want to.
Umineko No Naku Koro Ni. EP5. Battler, you may have forgot, but you were supposed to deny witches, not become one.
In the final installment of Xenosaga, one of the many antagonists, Dmitri Yuriev, tries to conquer his fear of God by becoming a god himself using the Zohar and Abel inside Omega Res Novae. It works, of course, but then his clone/son who is supposed to be dead, Albedo, pops up and takes it all away. Makes one wonder...
It's been speculated by some that the Zombies and Skeletons that spawn with armor in Minecraft used to be other miners that were there before you. Doesn't help that you can encounter abandoned mineshafts throughout the world.
The end of Zombie Driver has the protagonist that ran over hundreds of zombie hordes turn into a zombie from an infected bite he got much earlier in the game.
Used as a game mechanic in ZombiU. You technically only have one life, and when killed, your character then becomes a zombie, and all of your inventory is left with them. If you die, you take control of a different survivor who has to follow the same Voice with an Internet Connection to survive, but also having to track down your predecessor, defeat them, and getting back your inventory. If you have an internet connection, you may also come across other players' fallen survivors.
If you take a long time to feed on delicious meat, the Demon Virus in Digital Devil Saga is going to take hold on your psyche and Shapeshifter Mode Lock you in demon form. At that point, the kindest thing that can be done for you is to bash your brains in before you start mindlessly eating others. In a misguided effort to do good, two named characters attempt to resist the hunger. Neither case ends well. There are other cases in which instead the virus was left to stew in the body of the infectees for a very long while, with the same result. For extra added horror, the beings the second category produces? Angels.
In Fallout, if you tell your Vault's location to the Super Mutant leaders, you get a Non Standard Game Over that involves being dipped in FEV and turned into a Super Mutant yourself.
Subverted in Fallout New Vegas: Old World Blues, where the Courier has their heart, brain, and spine replaced with cybernetic parts just like the Lobotomites, but retains a psychic connection with their original brain.
In the Web GameI Saw Her Standing There the love of your life has been turned into a zombie. By the end of the game she turns you into a zombie. The epilogue states you're very happy together and that the two of you made pancakes.
Web Comics
In The Order of the Stick, the character Durkon Thundershield is shown to dislike the undead very strongly. During the events in Girard's dungeon, he is killed and reanimated as a vampire.
Web Original
Attempted by Chief in Arby 'n' the Chief, when his Machinima character, also named Jon, becomes an alien after fighting in a war against them. The effect was not as he intended.
A skit from Those Aren't Muskets featured on Cracked, "Dealing with the Guy who's Clearly Hiding a Zombie Bite" ended as more and more members of the group revealed that they'd also been bitten by zombies, until the couple who hadn't been agreed to be bitten so as not to be left out. Of course as it turned out the original guy had also been bitten by a vampire and a werewolf. In the end, we had a group of three zombies, a werezompyre and a mummy werezompyre that also happens to be a criminally insane android with holographic legs and a webshow. Michael Swaim can officially give Cyborg Pirate Ninja Jesus a run for his money.
Herobrine ends with George Smith becoming a new Herobrine.
The Red vs. Blue: The Recollection trilogy does this three times. The Meta, a shell of a man now playing host to a group of rogue AIs, was once part of the project that produced them; Pvt. Church, who constantly complains about the AIs and the trouble they cause, is revealed to be not only an AI himself (the Alpha), but the source of all the others; and finally, the Alpha was based on the mind of the Director of Project Freelancer, Doctor Leonard Church.
Finn and Jake of Adventure Time are both turned into Lumpy Space People by the end of "Trouble in Lumpy Space". They got better.
Also in the Adventure Time episode "Mortal Folly/Mortal Recoil" Princess Bubblegum sends Finn and Jake to fight the Lich. In the end Bubblegum is turned into the Lich's vessel. She too, gets better (sorta).
Parodied in Invader Zim in the episode "Bolognius Maximus" in which the title character and Dib transform into bologna. The DVD Commentary hangs a lampshade on this.
In the South Park episode "Ginger Kids," after Cartman starts a crusade against gingers, the other boys try to stop him by dying his hair red and using makeup to turn his skin pale and freckly. But then Cartman just changes his tune, starts a "ginger power" movement, and tries to wipe out all non-gingers. He stops and begins preaching tolerance after Kyle explains to him that he's not really a ginger.
Which later becomes ironic after "201" when Cartman finds out from his rival, Scott Tenorman, that he is his half brother through Scott's father (Cartman'a mom sleeping around, etc) whom Cartman had killed to get back at him. Both Scott and his father are gingers meaning Cartman is half-ginger.
