Vampire Hunter D: Being made a vampire in this world is like this, even for vampire hunters! In Bloodlust the eldest of the vampire hunting brothers is turned, and he instantly sides with Carmilla and threatens to kill a former associate by drinking her dry.
Possibly With Great Power Comes Great Insanity, thanks to what can happen with Dhampirs, or what happened in Book 2, where a person was in fact a 'latent' vampire. She was actually surprisingly reasonable, but with a complete personality change afterwards.
Or in The Rose Princess A woman with an axe to grind against the titular Vampiress (said vampiress had killed her family) got turned in an unusual fashion. After D killed the vampiress, Elena asked D to join her in ruling over the humans, and comments that being a vampire is so much better. There's quite a bit of insight given into her psychology, though. Averted with a later human, who resisted/rejected it.
In fact, it gets a lampshading/explanation in Book 1...there's debate over whether someone being turned is murder, as the victim is still around, but the personality changes are severe.
Darker Than Black: Tania has a quick and jarring her personality shift upon becoming a Contractor. Originally, she was a kind person. She immediately becomes cold towards her friends and by the third episode is really enthusiastic about the idea of selling her friends out to an intelligence agency for a chance for promotion. she even brutally kills her childhood friend/crush Nika. The series also has several examples of Contractors who don't turn on their loved ones, so it's possible that kind people like Tania are actually more liable to turn to brutality and coldness upon suddenly becoming detached from their emotions, while a person who was already detached may not have quite as jarring a shift.
Similar to the Vampire Hunter D example above, vampires spawned by other vampires in JoJo's Bizarre Adventure. One scene has a woman pleading with Dio to spare her baby, and he agrees, on the condition that he can drink her and turn her into a vampire. Immediately after the transformation, she tears her own baby apart.
In Blassreiter, Wolf Goring, the commander of the XAT, is infected with nanomachines and transforms into an Amalgam. Beatrice convinces him that this is a good thing, as only the Demoniacs will survive the coming Armageddon, and he sets to work infecting the rest of the XAT, all the while claiming it's for their own good.
Because every demon in Berserkwas once human, there's a good amount of this going around in the general universe. The Godhand picks their own by demanding that the demon-to-be sacrifice whoever or whatever he or she most cares about. Once someone becomes a demon, they usually cast off their humanity, with many of them engaging in eating their former species and/or raping people on the side. There are exceptions to the general norm (Zodd, Locus, and some others), but many demons in Berserk are dedicated to spreading misery and suffering among humanity, which is exactly how the Godhand likes it. It does help that the humans chosen to become demons by the Godhand were picked specifically because they were the type most likely to accept the Godhand's offer.
The majority of vampires in Shiki, though not all of them are happy about having to. Any who choose not to engage in this trope stand out as a result.
Marvel Zombies: A disturbing example shows up. In the worst Mad Scientist fashion, after Reed Richards sees a zombie She-Hulk eat his children he decides (after studying the zombies) that they are the next evolutionary step and turns the whole FF into flesh eating fiends! Also from the same book, Zombie Infectee Giant Man knocks out and hides Black Panther because he wants to have a "snack" to eat later, knowing his former colleagues would kill the world's population in days. He slowly amputates T'challa's limbs and eats them to stave off his hunger. He survives, thankfully, and gets a new leg out of it.
In Evil Evolution it's suggested that Reed is inadvertently to blame for unleashing the virus upon his universe to begin with, thus piling guilt issues on a genocidal scale on top of everything else as well.
Zombie Spider-Man was hit with the business end of the Horror Hunger stick just as he got home to Mary Jane and Aunt May. Hilarity ensued.
Being vamped in 30 Days of Night pretty much instantly makes you a sociopath. To date only three people have had the moral strength to resist this, everyone else basically gives in to their inner jerk.
In Swamp Thing: The Curse (part of Alan Moore's run), a recently-vamped boy taunts one of his parents about how they were too ineffectual to protect him while he tears out the other's neck.
In Transmetropolitan, both ends of the spectrum are represented: Fred Christ's Half-Human Hybrid Transient Gray aliens are all-too-quick to embrace their newly-bought alien side and their role as a vengeful, oppressed minority. On the other hand, Tico and other "foglets" are living it up as nanomachine clouds, holding no ill will towards mankind. And then it's stated that Transients draw most of their recruits from humans who already have alienation issues, making this possibly an Invoked Trope.
Averted in X-Men #2. It's made abundantly clear in the text that, despite this trope appearing to be in play, there is definitely brainwashing in play, at the least More Than Mind Control.
Then played straight later-on, when people are turned in the traditional way.
Lucy Westerna in the DC comic Victorian Undead 2: Sherlock Holmes vs Dracula. In this alternate version of Dracula, she avoids being staked upon her awakening after being turned and sides with Dracula to takes over London. She goes on and on to the protagonists how she likes the power that being a vampire has given her Then its later revealed she's not very loyal to Dracula either, leaving in the middle of the conflict with her newfound power rather than be killed.
And in the second half of the story, Jade kidnaps Viper and force feeds her Ikazuki's chi (killing him in the process), transforming her into Hebi, the new General of the Samurai Khan, who is completely loyal to Jade.. Justified, as Jade specifically designed the ritual that caused this change to corrupt Viper.
In The Swarm of War, what used to be James (and is now the new Overmind) and its transformed human servants care little for humanity.
Halkegenia Online. Ephilates is quite pleased with being turned into a faerie, and refuses to tolerate the faerie lords' attempts to return to earth. To that end, he allies with the conspiracy to overthrow the crown.
In many The Conversion Bureau stories, those who are ponified end up renouncing their human past, if not outright working against it.
Averted in Let the Right One In. Eli feeds on people, and will often use the fact that she's looks like a 12-13 year old girl to appear harmless. However, she clearly does not enjoy doing it and only does so when she can no longer resist the hunger. After her first shown attack in the film she looks like she's about to cry afterwards. The only time she deliberately unleashes death is At the end when the four bullies are attempting to drown Oskar. This clearly hits her Berserk Button and launches an Unstoppable Rage.
Pretty much every vampire in the Blade Trilogy, whether five minutes or five centuries old, is invariably fine with messily killing humans. Vampirism comes with a nearly insatiable thirst for human blood. The handful of exceptions were all cured with an anti-vampire drug, and of course Blade himself (being a Daywalker) is the big exception. Blade did feed on humans at one point, mostly homeless people, but that was before meeting Whistler and developing the thirst-suppressing serum.
Though any movie with still sentient zombies has this, The Return Of The Living Dead shows how Freddy 'has an epiphany' that the only thing that can stop his pain and will show Tina's true love for him, is letting him eat her delicious brains.
The hemophages in Ultraviolet follow the "race war" scenario, but since the humans treat them like a plague to be eradicated, it's not like they have a lot of options. At the beginning, two investigators arrive to the scene of the crime, where a hemophage's body can be seen. One of them accidentally touches the blood. Two seconds later, his partner pulls out a gun and puts a bullet in his head (which is, of course, ignoring the fact that he's bound to get some of his partner's blood on him).
A heroic example occurs in Avatar. As Jake gets used to his new body, he also shifts loyalties towards the alien race, and he eventually leads them to war against the humans. And by the end, he gives up his human body to stay with the Na'Vi. So do all the other Avatars, for that matter.
Anyone bitten in the From Dusk Till Dawn series. The infection is more akin to a zombie bite and once the victim turns, the only thing on their mind is attacking anyone human for their blood (which is a very messy process since these vampires tend to rip off limbs) or bring into their ranks.
Magneto tries to invoke a sympathy version of this by turning all the world leaders into mutants in X-Men, it being less "I'm going to make humanity my bitch!" and more "I'd better pull back on the anti-mutant rhetoric and policies since I'm one now."
In the Count Yorga movies all the turned victims instantly become evil and obey Yorga without question examples being Erica, who attacks Hayes along with the other brides, tries to attack Michael when he comes across the dying Hayes (and yet oddly gives him a chance to stake her, he doesn't go through with it) and attacking Michael and Donna as they're leaving right after Yorga is killed. Showing that the vampirism is permanent and she is beyond saving, plus that the vampires are inherently evil. We also find out that the blonde haired bride of Yorga's is Donna's mother (who at the beginning had "mysteriously died") who likewise attacks Dr. Hayes with the other brides. An unused scene also showcased she had become monstrous enough to eat ababy◊. Donna herself turns at the end of the film and instantly attack Michael when his guard is down.
In the sequel, theres the Cynthia's sister, Ellen, who joins Yorga's brides after his harem attack their family. She, along with the other brides, torments Cynthia mentally as she trying to find her way out of Yorga's mansion. She is also part of an ambush on her boyfriend which results in him killed and thrown to the vampire brides for food (of which Ellen joins in on). At the end, one of the heroes, Balwin, succumbs to vampirism after Yorga is killed and instantly claims Cynthia as his own victim right after he saves her.
Lillian and Mara in the first film of the Subspecies. Mara is the first victim of Radu's, after being buried and rising as an undead later, helps Radu lure out her friend Michelle and capture her. Lillian is kidnapped in the middle of the film and slowly turned by Radu. We actually see her becoming a monster as her transformation finishes while Michelle and she are trying to escape Radu's castle. Lillian instantly turns on Michelle once her fangs form, scaring her right into the arms of Mara. The next scene sees the two dragging Michelle to Radu so he can bite her and even smiling wickedly as they hold her down so she can't escape, pretty much solidifying their loss of humanity. Radu succeeds in turning Michelle but she mostly flip flops on this trope through the series, trying to fight from becoming an all out monster like Radu is though falls off the wagon now and then due to his influence.
Horror of Dracula: Lucy , after becoming a vampire, preys on her niece as one of her first victims.
The Brides of Dracula: A village girl and a school teacher, Gina, fall victim to the vampire Baron and instantly become evil upon resurrecting. Though oddly the Baron's mother doesn't succumb to this trope when she is turned.
Taste the Blood of Dracula: Another character named Lucy is turned and helps Dracula and her hypnotized friend killed her father and later bites her boyfriend.
The Satanic Rites of Dracula: All the women in Dracula's cellar were once agents investigating the cult a company had ties with and were captured and turned. In the film we see the secretary of one of the agents, Jane, kidnapped herself and turned by Dracula. When Jessica discovers her in the cellar and tries to free her. Jane bears her fangs and tries to bite her, helping the vampire women knock her down so they can feed on her. She also tries to pull the same trick on her boss as well.
In Dracula 2000, gives us the three Dracula's brides of the film. While the first two are somewhat dubious (Solina was a thief as a human and seem to gleefully relish being a vampire, and we don't know much about Valrie as she just a reporter in the wrong place at the wrong time), Lucy fits the trope well as she was a roommate of Drac's main target Mary and the two got along well. Once she was turned however, she had no reservations on attacking Mary with the other two brides.
This is thought to be the case with Will's nanomachine-augmented hybrids in Transcendence. But it's not.
Edmond Hamilton's The Man Who Evolved might be the Ur Example. A man invents a machine that accelerates his evolution. The first two stages, he is still somewhat decent in behavior. In the third stage, he states he intends to take over Earth and use it as a laboratory. The protagonist stops him by convincing him to evolve further. It works; the next stage is beyond such desires.
Usually averted by Alastair Reynolds, but his most recent Revelation Space prequel, The Prefect, includes Aurora, the only survivor of the eighty experimental subjects who were uploaded into computers. Some trauma was involved (like seeing all her fellow Transmigrants freeze or crash), but basically she decides her survival trumps everyone else's and plots to kill everyone in the Glitter Band habitats for safety.
In The Dresden Files all the vampire courts appear to have this to a greater or lesser degree. When the Red Court fully transform the human part of them dies entirely, though the physical form is still used as a "flesh mask" by the creature within. However they don't cross over until they kill a human and drink their blood, and several members of the organization dedicated to fighting the Red Court are Vampire Refugees. The Black Court are Dracula style vampires, but little is mentioned about how they reproduce except that they do it fast, which does imply a pretty complete transformation. The White Court are the least hit by it. They end up sharing their body with a demonic Hunger that heavily influences them, but it is possible to fight against it. They're also a family, so it's more a matter of accepting the family business than a 180 degree allegiance switch.
In Known Space if an ape or hominid at a certain age eats from the Tree of Life it becomes a Protector (too young, and it doesn't do anything; too old and they don't survive the metamorphosis) where they enter true adulthood and have a massive amount of changes, making it stronger, faster, more resistant to damage, and granting it super-human intelligence. A Protector is compelled to protect his genetic lineage, and his species from any possible harm. As a matter of course, Protectors exterminate any nearby alien species on the grounds that they might become a threat.
Most Protectors are only interested in protecting their direct descendants and are perfectly willing to nuke other human groups to do so. It's a rare Protector who can make itself feel concerned for humanity as a whole, and usually only either when all of its own lineage has been killed off, or when it realizes that helping humanity as a whole is the best way to help its own bloodline. Word of God says that the more intelligent a Protector was as a breeder, the easily it can make the jump to protecting everybody and not just its own children.
Inverted in The Vampire Files with Whitey Kroun, who'd actually been prone to rape and murder women while drugged out of his mind prior to becoming a vampire. His transformation rendered him unable to do drugs anymore, and brain trauma suffered when he died stripped away his memories of his past misdeeds and their motives, allowing him to achieve a Heel-Face Turnbecause he'd ceased being human.
In Oleg Divov's Brothers in Reason, the Big Bad turns out to be a result of a Soviet experiment to turn 1000 children into psychics; only half-a-dozen children survived the process. He is one of the three to actually get enormous Psychic Powers. Subverted in that the two other super-psychics as well as numerous other lesser ones stayed true to their ideals.
In James H Schmitz's "The Machmen", the machmen claim this is the case, but the protagonist believes they are merely being brainwashed as well as enhanced.
Inverted by Myria Lejean in Thief of Time: rather than a human made into a monster choosing to be a monster, she is something inhuman made into a human that ends up choosing humanity's side. (Humanity Is Infectious, but the other incarnating Auditors merely go insane from it).
In the Immortals After Dark series by Kresley Cole, Webb is a fanatic bent on destroying immortals, believing them to be unnatural. He becomes immortal himself, and promptly begins plotting the subjugation of the human race he championed, having decided that he's now part of a master race or some such. Really, it's just a predilection of his; the only thing that changes is which team he's on.
Live Action TV
Lauren on Being Human is less than humane. Mitchell transformed her into a vampire to "save her" from dying when he fed from her and has regretted it ever since, not least because the once sweet and kind Lauren has taken to her vampiric biological mandate with gusto and sociopathy. It remains to be see whether she changes her mind or doesn't get killed first.
One of Riley's friends gets hybridized with demon body parts by Adam against his will. Despite being a trained and loyal soldier involved in a demon hunting/capturing program, he completely switches sides after the operation because he liked the power. That, and had mind control chips in his head, as well as being dead and re-animated.
All the vampires. Instantly. Although "you're not looking at your friend. You're looking at the thing that killed him," is the official story from the start, this gets... layered over time. Because vampires may have had their souls replaced by demons and a complete moral rewrite, but they still have the same memory set, and in some cases this hits harder than others.
For example, Darla in the flashbacks on Angel was quite pleased with Angelus' decision to kill off his entire hometown to one-up her suggestion that he do the normal thing and kill his loved ones. After he got to his family, she took great pleasure in pointing out how yes, he was evil now, but he was still him, and now his father could never approve of him, being dead. This hit home.
And William/Spike does exactly this, and there's Vampire Willow. Theories exist that the vampire character derives from everything suppressed by the original human, but this only applies in some cases. Harmony, for example, starts killing people and decides she wants to be some kind of dark lord, but honestly doesn't change at all. It probably has something to do with self-image. People with low self-esteem seem to change more, but the completely pathetic seem to stay pathetic. That Liam and William seemed like losers but turned into Badass vampires probably indicates that they had Hidden Depths.
Doyle went through a huge personal crisis when he found out he was half-demon and started being able to change, and he did go from an upstanding third-grade-teacher married to a woman he met volunteering at a soup kitchen to a faintly skeezy always-in-debt skater around the edges of legality in the demon side of L.A., but that wasn't his heritage. Just his emotional reaction.
Being a werewolf, Veruca feels sorry for normal humans, because they will never feel as alive and free as she does. She lives by her instincts (which include killing), simply because she sees it as a natural thing to do.
All the Psychic Children we meet who aren't killed before they can except Sam, who never intentionally betrays humanity turn their demon-blood-born power toward Azazel's murderous expectations. Jake even kills Sam and unleashes hell on earth, intending to lead an army of demons against humanity.
Gordon Walker was already an antagonist before he got turned into a vampire, with the goal of killing Sam because he believed Sam was the Anti Christ and would lead hell's army against humanity. While a turned Gordan (says he) planned to kill himself after killing Sam, along the way he killed to feed, killed newly-made vampires, killed the hunter he was working with, nearly killed Dean, and turned a young woman as a distraction.
The Synthetics in Odyssey 5 are either completely synthetically grown Artificial Humans or nanomachine enhanced humans. The latter have to obey the AIs which created the nanites, being even part of a low-grade Hive Mind; however Chuck Taggart reveals after reversing his assimilation that the process can't completely subsume a human unless they want it to. The terrifying implication being that those humans infected want to serve the genocidal AIs.
When Tory discovered she was a Cylon on Battlestar Galactica, in contrast to Sam, Saul, and Galen, who went all What Have I Become?, it didn't take her long to start seeing the benefits of it and how it meant she was 'better' than humanity, something the ordinary Cylons no longer really boast about anymore though they did use to. She even advised actions that would likely have caused thousands of humans to die in the mid season finale. All of which makes little sense given the Cylons she was with were actually in a weaker position than the humans at that point. In light of what happened later, maybe she should have waited a while before throwing her eggs into another basket.
Evil industrialist and Diabolical Mastermind Tobias Vaughn from "The Invasion" has undergone partial cyber-conversion by the Cybermen but retains his emotions and appears outwardly human.
In "The Next Doctor", the Cybermen attempt to make their collaborator Miss Hartigan their "King", but she turns out to be exceptionally strong-willed - while most of her morality gets discarded, she retains her emotions and is able to take control of the Cybermen's Hive Mind. The Doctor defeats her by breaking her link to the Cybermen's programming, causing her to see what she had become and destroy both the Cybermen and herself.
In the Ark in Space, one of the crew gets infected by the Wirrn, and is turned into one of them. He's also the one that causes the ship to blow up be after the Doctor tampered with it, and he theorizes he must've had just enough humanity left in him to resist the Wirrn's hive mind.
Inverted by Dalek Sec, who exhibits signs of Trans-Dalek Treachery after merging with a human. Humanity Is Infectious.
A staple of the Kamen Rider franchise. Where the many of the villains were once human but have given it up due to their season's power sources. Of course the heroes usually avert this getting their own powers from the same place.
The short-lived series Prey has a new species evolving from humanity and living in secret among us. These Homo Dominants (as they're named later) lack any emotions and have an instinctive desire to dominate and/or destroy humans, believing that, otherwise, the much more numerous humans will destroy them just like we killed the Neanderthals. They are stronger, faster, and smarter than most humans (one episode shows they have mastered nanotechnology). The protagonists are a group of scientists who discover the existence of the Dominants and fight to stay alive and reveal the truth to the world (with the help of a Dominant who has seconds thoughts). After the revelation, the government, as expected, starts trying to eliminate the threat to humanity by striking first. Peace talks between human and Dominant representatives are disrupted by radical elements. Both sides attempt to figure out ways of turning one species into another (humans try it with a drug, while Dominants create a nanovirus that rewrites DNA).
The series Teen Wolf has this. The fullness of the moon triggers a shift in personality for werewolves, which can be overcome by experience (Derek) or a special focus (Scott). Then, there are cases like Isaac, Erica, and Boyd, who are generally more bully-like regardless of the lunar phases.
Clan Tzimisce of Vampire: The Masquerade deserves special mention; all vampires in the setting suffer from humanity degeneration, but the Tzimisce have explicitly become transhumanists after developing Vicissitude. They tend to lose their humanity quite a bit faster than average.
Humans vs. Zombies breathes this trope; once tagged, you become a Zombie, and you need to feed. Of course, that doesn't mean you still can't have a vendetta against the other humans who were less than helpful in keeping you safe...
In Paranoia, the human membership of the Corpore Metal secret society works to increase the prevalence of cybernetics in the hope of eventually uploading every human into a superior robot body. The bot membership figures there are enough bots to run Alpha Complex already, and simply exterminating the humans and taking over would be lots easier. Corpore Metallics who manage to completely replace their organic parts with cyber, effectively becoming bots, invariably side with the latter group.
The same is true to a lesser extent with biological transhumans. Mutants with psionic powers are eagerly recruited into the Psion conspiracy, one of the few secret societies that does pose a legitimate threat to Alpha Complex.
Eclipse Phase has exhumans, people who modified their bodies and minds too much and became utterly alien to the rest of transhumanity in the process.
Warhammer 40,000: The Traitor Legions were half of the original Space Marine Legions, the half who followed their Primarch fathers as they rebelled against the Emperor and turned to Chaos for various reasons during the Horus Heresy, becoming the first Chaos Space Marines. While most of the mortal forces of Chaos are comprised of the humans, mutants, later founding and some loyalist marines who decided to join their ranks, the original Legions remain amongst the most powerful and respected forces of Chaos.
Starcraft: Kerrigan is Queen Bitch of this trope. At first she's somewhat mind controlled, yes, but also fairly autonomous. And then she's entirely autonomous and sells out the human race to lead the Zerg. She seemed slightly sad about trying to kill Raynor out but she got better. Apparently tied to the very nasty mental conditioning and childhood she had.
When she infests Ethan Stewart, he becomes an intelligent and fairly free-willed Zerg-Terran (and she knows it, so she threatens him if he ever turns against her). He immediately falls in love with her and follows her orders and does everything to gain her approval. It doesn't work.StarcraftII goes into a bit more detail, and the Zerg Campaign will likely go into more...
By the end of the plot of Heart of the Swarm, it's become complicated. The Xel'Naga began molding both the Protoss and the Zerg into what they are now, apparently because it's just how their species perpetuates themselves. But their work on the Zerg was subverted by Amon, the Dark Voice, who gave the Zerg a biological imperative to obey him and destroy the Protoss. The Overmind freed the Zerg from that by getting himself killed so Kerrigan could take over. But it turns out that even when freed from Amon's influence, the Zerg are still a Horde of Alien Locusts. Once freed, Kerrigan is relatively humane and an Anti-Villain, but still more inhuman than she was before.
Ironically, the protagonist still becomes what he hates in the process. He loathes them for trying to turn him into a murderously deranged cyborg... and by the end, he has become just that — only directed at his creators, casually killing anyone even slightly related to his capture and transformation.
Half-Life 2: The Transhuman Arm of the Combine Overwatch combine this with being Les Collaborateurs. Humans start out joining Civil Protection to get decent rations; promotions are tied to voluntary brainwashing until they become eligible for Mind Rape and modification into actual transhumans (though the Combine often picks up random civilians for conversion too). Soldiers presumably go through the same rank system with the highest-ranking Elites being more like synths than cyborgs.
Mass Effect 3: Javik explains that this happened to a species during his time: the Zha'til. They resorted to cybernetic implants to save themselves, and wound up being controlled by the implants and changing into a synthetic race which waged war on the rest of the galaxy. They are one of the main reasons Javik hates AI's so much.
Cerberus's use of indoctrinated soldiers and the Illusive Man's plan to make humans the dominant species via control of The Reapers are other examples. Emphasis on treachery as TIM ends up indoctrinated by the endgame.
Played with in Mass Effect 2. In the Overlord DLC, because of the extensive cybernetic implants that Shepard received from the Lazarus Project that brought them back to life, this was enough to allow them to become temporarily hacked by David Archer. This doesn't actually stop Shepard however, but simply lets them see the world in Matrix Raining Code.
Shin Megami TenseiHalf-Human HybridFusion Dancesalways end like this. The interested parties seldom lose their identities, but become quickly enchanted with their new fancy powers and start allowing their fanaticism and ego to dictate their actions. It's more a case where every negative emotion is amplified by the simple fact the newly minted hybrid is so drunk on power he or she no longer bothers to answer to any problem he or she can't deal with by simple power.
In the supplementary material for Sword of the Stars, this is the case with the Locusts. They were once an organic race until they invented Brain Uploading, upon which some of those who became engrams decided they were superior to the baselines and bombed the latter back to the stone age before setting out into the void to make more of themselves while wiping out the inferiors.
Touhou: This is the reason why Byakuren was sealed in another dimension. In her pursuit of immortality via black magic, she begins to side with the youkai, while previously she was a venerable nun. Her youkai followers treat her as a Messianic Archetype; the humans won't.
On a lesser note, fellow black magicians Patchouli Knowledge and Alice Margatroid were never fond of humans to begin with.
Alice is stated to be antisocial and unpleasant, but takes politeness and Sacred Hospitality as seriously as most of Gensokyo's denizens. Patchouli is a total aversion of this trope, since she was born a member of the Witch Species rather than being a former human like Alice or Byakuren.
Prototype 2 has this in spades, for all major Infected characters. When it comes to the Evolved, a super-strain of infected that retain their intelligence and "humanity" while gaining combat and infiltration skills. Later in the story, a chemical has a chance to turn those nearby into Evolved if released, and those transformed will turn on their former allies. These even applies to Blackwatch soldiers, who take containment beyond seriously.
Averted heavily in X Com Enemy Unknown. The Volunteer is pretty much a Physical God by the end, and could have saved himself/herself and ruled the rest of the galaxy as a God. Instead they choose to sacrifice themselves to save Earth after the Temple Ship goes critical.
Brock from Braceface Fangface and Brock Of The Undead. Despite now being a walking corpse that must feed on blood who must obey his new master. The power more then outweighs the negetives and he happyily uses it to bite his girlfriend, Maria, and bring her into the ranks of the undead.
In the Metamor City podcast novel Making The Cut, this happens to Miriam after she's turned into a vampire. The vampires order her (which cannot be disobeyed) to not kill herself, do anything against them, lie to them, and she has to operate as a spy for them. They eliminate every possible option for rebellion/escape, or letting her Psi Collective people know what happened to her in any way. They miss one or two tiny loopholes, which she exploits for all they are worth in an attempt to save her allies.
The Jonathan Coulton song "Re: Your Brains" has this as its central premise, with the singer using office lingo while attempting to talk a (former) co-worker into allowing himself and the other zombies in to eat the survivors' brains.
In the site Vamp You, all vampire transformations work like this. Anyone who is turned, gets a villain card.
Orion's Arm presents the phenomenon of 'hyperautism', a condition that arises following the ascension of a mind to one of higher toposophic level with the result that it can fully comprehend and model the mental and emotional states of its former peers. They effectively lose the ability to see less capable beings as sentients... instead envisaging them as simple automatons or even extensions of their own minds. This can result in Hyperautistic Sociopathy when the transcendant mind sees its former peers as tools to be used and abused as it sees fit.