Though undeniably horrifying and on occasion dangerous to human beings, there's nothing objectively evil in what parasites or parasitoids do. Regardless of whether they control the minds of their hosts, use them as incubators for their young, simply eat them (or their nutrients) from within or just generally make their lives a little bit more uncomfortable than normal, the creature committing these acts is merely trying to survive and reproduce in much the same way as any other animal.
However, humans have a nasty habit of projecting their morality on the natural world, or at the very least, forgetting that Nature Is Not Nice. Consequently, parasitism in nature may be regarded as immoral and vile. Imagine someone living in your house without your consent, or stealing your stuff. That's what parasites are.
Thusly, fictional parasites, parasitoids and their ilk can all too often be depicted as evil, with their need to live upon others being regarded as cruel or immoral especially if sentient. In keeping with the Always Chaotic Evil portrayal, dysfunctional societies with lots of backstabbings and betrayals may be a possibility. If sufficiently powerful and/or intelligent, they might even qualify as the main villains of the story.
For this reason, parasites may end up becoming the exception to a hero's code against killing, even if they do qualify as sapient life forms; in extreme cases, they may even become the target of a Guilt-Free Extermination War.
NOTE: To qualify for this trope, the parasite or parasitoid must be confirmed to be of evil origins or deliberately malevolent. An ordinary parasite simply acting in its own natural interests does not count.
- Dragon Ball GT: The machine mutant Baby is a parasitic lifeform that invades the bodies of his targets via scratches, completely taking over their mind and altering their appearance as well as being able to use his host's abilities as his own. A Psychopathic Manchild by nature, he takes over Vegeta's body upon arriving on Earth and uses his powers to enslave and brainwash nearly everyone on the planet.
- Parasyte plays with this trope extensively:
- The titular parasites may be sapient creatures who see their human hosts as nothing more than livestock to kill, devour, and impersonate. But they are not Always Chaotic Evil for it, nor are they treated as such by the story; instead, they operate under Blue-and-Orange Morality, at best being logical, ruthlessly pragmatic beings who only do what they do because it is how they survive as a species in the first place. Some chose to engage in symbiosis with ther hosts, but only when they are forced to do so in order to survive, and some even learn human emotions and empathy over time. In fact, the only outright evil characters seen throughout the series are a handful of ordinary humans.
- Shinichi, the protagonist of the series, even goes as far as to call this trope into question, deciding at one point that humans have no right to kill or impose their values on the parasites because of the above facts. Though he later grows out of this mindset, he still doesn't hold any malice against the parasites he fights and protects people from.
- Yu-Gi-Oh! ARC-V: The Doctor brainwashes his victims by inserting parasitic insects in their ears. The infectees become uncharacteristically sadistic, rejoicing when watching others suffer and even taunting their friends when they try to set them free.
- Big Finish Doctor Who: In "Enemy of the Daleks", the the Kiseibya were created as a biological weapon for use in the war against the Daleks, they are voracious parasitoids in their larval stage, slowly and painfully devouring their hosts from the inside before metamorphosing into metal-eating adults. They're found to be intelligent enough to speak, but possessed of an outlook that's nothing short of terrifying: if they were deprived of Daleks to prey on for food and incubation, they'd gladly turn on humanity. After seeing the Kiseibya disable the Black Dalek and implant him with their eggs, the Doctor decides that they can't be allowed to leave their home world...
- Supergod: Morrigan Lugus is a godlike colony of parasitoid fungus built around the bodies of three long-dead astronauts, and easily the most malevolent of the gods appearing in this story. Along with his power to infest anyone in breathing range and make them worship it until they die of fungal growths in their lungs, Lugus actually gets a speech in which he gloats over the weaknesses of humanity, decrying its worthlessness for still obsessing over gods. However, Reddin hopes that the minds of the astronauts are still present somewhere within Lugus, and can used to inspire the Eldritch Abomination to save the human race. As it turns out, the astronauts are aware that they were sacrificed for the sake of an experiment, and want revenge: it's for this reason that Lugus ultimately betrays his creators in order to sporulate unfettered across the world, condemning the human race to extinction.
- A Little Too Much of a Good Thing, a Gravity Falls fanfic, features a race of insect-fungus hybrids known as Forger Wasps. Beginning with a single parasitic Hive Queen that inhabits a host (in this case, Mabel) for life, she spreads eggs to everyone the host touches; the offspring that hatch are parasitoids that transform their victims into perfect replicas of the Queen's host — give or take some super-strength — gradually destroying their minds in the process. The parasitoid clones are uniformly portrayed as creepy, sadistic Stepford Smiler types, while the Queen is depicted as a cold-hearted, uncaring monster with ambitions of taking over the world — and using the intelligence of her most brilliant offspring to condemn Mabel to a hellish eternal life, just so the Queen can attain immortality along with her.
- Alien: The Xenomorph is a parasitoid lifeform that was created after an insane A.I. repeatedly experimented on various creatures by exposing them to a extremely powerful mutagen. As a bioweapon, the species possesses brutal cunning, but no empathy and no overt goal except to expand, killing other beings even when it doesn't intend to use them as hosts. This particular parasitoid is so vile that genocide is considered a worthwhile alternate to letting them roam free, while in Superman/Aliens II: God War, Superman himself states that the Xenomorphs are a disease that has no right to exist and lives only to annihilate.
- Slither: The alien parasite is a galactic menace that travels from planet to planet, destroying anything in its path. Its modus operandi features it merging with a host from the local population and then begin reproducing by brutally impregnating unsuspecting hosts, infesting all forms of life with its slug-like offspring and absorbing the infested creatures into itself until it's essentially a living biosphere. When it merges with Grant Grant's mind at the beginning of the story, the resulting monster not only murders pets across the town for their meat, violently impregnates Brenda and infests just about everyone in town with the brain slugs, but also demonstrates the very worst of Grant's own attitude problems: jealousy, controlling tendencies, and a vicious temper.
- The Thing (1982): The titular villain is a microscopic alien parasite that can corrupt and assimilate any living being it comes into contact with. Throughout the film, it actively instils fear and paranoia on the heroes, turning them against each other to make them easier targets. In the climax, it's revealed that one of its "selves" was in charge of building a small spacecraft, implying that the alien intended to spread the infection across the entire Earth. The tie-in story The Things confirms that the Thing is sentient, albeit nightmarishly abstract in thought patterns, and regards human intelligence as so disgusting that it's prepared to assimilate everyone on Earth to eradicate it and assimilation is a process that its victims directly compare to rape.
- Venom (2018): Invoked, Venom is constantly furious and offended at being called a "parasite". Considering he genuinely believes in having a symbiotic relationship with Eddie (unlike other members of his kind), it's understandable.
- Animorphs: The Yeerks are a race of parasitic slugs controlling the minds of those they inhabit; they are the principal antagonists of the series, and have built a galactic empire out of their need to find new bodies in which to live. Yeerk infestation is often portrayed as a terrifying, dehumanizing experience, and the Yeerks themselves are encouraged to treat their hosts as little more than cattle — to the point of psychologically torturing them in order to force them to submit. However, the portrayal becomes much more complex as the series continues, especially once it becomes clear that the Yeerk Empire is harbouring a freedom movement that wants to shift the species away from parasitism and into symbiosis; these rebel Yeerks are regarded as sympathetic and even serve as tentative allies of the Animorphs on occasion.
- The Cancer Cowboy Rides, a John Connolly short story, plays with this. The source of Buddy Carson's cancer-spreading touch is imagined as a black worm living deep within him, encouraging him to infect more people as time goes on. For good measure, it's not above torturing him from the inside if he ever begins to weaken in his devotion to propagating "The Black Word." However, Carson isn't even sure that the worm is real or if it's just a delusion he suffers — and even if it exists, the cowardly evangelistic horror within him is still less villainous than Carson himself.
- Known Space: Played with. The puppeteers are embarrassed that they're parasitoids, requiring a livestock animal they infest with their larvae to propagate (which they insist is actually the third sex of their species). However, their extreme cowardice leads them to do such things as socially engineering their rivals on the galactic stage into ineffectuality and release metal-eating bioweapons (a type of yeast that feeds on room-temperature superconductors) into Pre Cursor ruins so they can't ever become a threat, never mind that they could study such Lost Technology.
- Necroscope: The Wamphyri walk a wobbly line between symbiosis and parasitism: essentially a race of alien leeches, they empower their hosts with immortality, inhuman strength, necromancy and the ability to warp flesh at a touch. Unfortunately, being bonded means being painfully and unwillingly impregnated with a fetal vampire symbiote complete with rape imagery - and subject to its hunger. Regardless of how benevolent they were beforehand, hosts are all gradually corrupted into villainy by their new appetites, to the point that any differences between the parasite and its host vanish: the end result is a Humanoid Abomination that will think nothing of committing murder, torture, anatomically-impossible rape, and other crimes too hideous to describe. Vampires are considered so evil in this setting that, upon dying, they're actually excluded by the other dead minds, who want nothing to do with them. Even Harry Keogh isn't immune to this sort of treatment once he gets infected by a vampire symbiote.
- Perdido Street Station: The Handlingers, on top of being cold-hearted Puppeteer Parasites that create their own shadow communities among other sentient races, are also supporters of Mayor Rudgutter's corrupt regime, serving as elite spies and operatives across the city.
- Hyperion Cantos: The TechnoCore are revealed in the last book to be a product of digital evolution which was massively inclined toward parasitism (like stealing the codes needed for reproduction). In 99% of the cases, their view of the humans is limited to either using them as Wetware CPU, or as DNA donors for their artificial bodies — including the A.I.s attempting to move beyond the purely parasitic mindset.
- Stargate SG-1: The Goa'uld are snake-like Puppeteer Parasites, and commonly use the powers they naturally confer on their hosts to pose as gods, especially when combined with their advanced (and stolen) technology. They regard other life-forms as nothing more than slaves, treating them with a mixture of sneering arrogance and chortling sadism, gladly leaving their hosts trapped in a tormented state of helplessness. It's made clear that they're biologically locked into this through Genetic Memory, with each new generation of Goa'uld being inclined to regard themselves as true gods and stab each other in the back to acquire more power. Only one Goa'uld has been confirmed to have given up the parasitic lifestyle: she founded the Tok'ra, a resistance movement advocating cooperation with their hosts; they're a much more pleasant people - though they're still a bit on the haughty side.
- Celebro, the main villain of Ultraman Z, is a space parasite that simply revels in causing death and destruction, having manipulated numerous planets and civilizations into creating super-weapons that results in their own destruction. He even have a name for his patented planetary destruction operation: The Civilization Self-Destruction Game!
- Ghlaunder, the Gossamer King, is a Chaotic Evil god of parasitism and disease strongly associated with biting insects — he himself takes the form of a hideous, mosquito-like monster. Thematically, his cult and mythos emphasize motifs of feeding off of others while spreading weakness and disease to one's victims.
- First edition's second Bestiary specifically notes that parasitic animals, such as lampreys or ticks, do not have counterparts among agathions, Neutral Good outsiders who resemble humanoid animals of various sorts. While the text notes that parasites are not intrinsically evil, it also states that their habits and natures are too far from the noble goals of the upper planes for blessed souls to wish to model themselves off of them.
- Halo: The Flood is a race of parasitic superorganisms yearning to usher in a perfect, utopia without classes, degradation, conflict, or unhappiness. And how do they aim to achieve this goal, you ask? Why by forcibly turning all organic lifeforms across the universe into hideous, disgusting zombies, of course! The The Forerunner Saga only amps up their villainy by dropping the bombshell that the Flood is a Precursor bioweapon birthed from their powered remains for the sole purpose of exacting their revenge upon the Forerunners.
- Kirby: Triple Deluxe: Word of God says that the Big Bad Queen Sectonia is based on parasitic wasps. Her parasitism, in her case, is her using her magic to repeatedly possess other beings, due to her obsession with beauty. Played with in that she used to be a good guy in the past, and that as other games revealed, she wasn't always a wasp, but something more like her subordinate Taranza (a spider-person).
- The Legend of Zelda:
- The Legend of Zelda: Ocarina of Time: In a gambit to obtain the Sage Stones, Ganondorf creates monsters to parasitize two of its guardians. Gohma's disease kills the Great Deku Tree, though Link manages to save Lord Jabu Jabu from Barinade's infection.
- The Legend of Zelda: Skyward Sword: The sky spirit Levias has been infected by Bilocyte, the ocular parasite. Under its influence, the deity turns malicious, summoning lightning storms wherever it goes and attacking Skyloftians on sight.
- Ōkami: The demon Blight parasitizes the Emperor's stomach, using his body to spread a toxic fog around the city.
- [PROTOTYPE]: The Supreme Hunter starts life as a parasitic cancer-like entity created to kill Alex Mercer; Mercer later re-purposes it for use against Elizabeth Greene, but after Greene's biology rejects the re-engineered virus, it mutates into a large shapeshifting humanoid Hunter obsessed with consuming its "father" - intelligent, manipulative, scheming and ultimately the final boss of the game.
- Resident Evil 7: Biohazard: E-001 has the rare distinction of being both an fungal parasite and a brood parasite: producing clouds of brain-warping spores, this mould takes over the brains of anyone in range and encourages them to see her as their daughter. In the process, the victims are gradually driven insane, often mutating hideously into monstrous toys for E-001 to exploit and abuse at will. Spiteful, manipulative and gleefully sadistic, E-001 is second only to Lucas in sheer malice, repaying the kindness of the Bakers with suffering and luring in Ethan for no other reason than to replace a toy she's grown tired of.
- The Secret World: The Filth is eventually revealed to be capable of infecting plantlife as well as humans, resulting in a wide variety of fungal monstrosities devoted to serving the Dreamers as loyally as any run-of-the-mill Filth infectee. Among these include examples of the parasitoid fungus Ophiocordyceps Unilateralis; they work much as they do in real life - except the results can be applied to humans. In this case, the parasite isn't just evil, it's a flat-out Lovecraftian horror.
- Trauma Center: The main threat in the DS games are the bioengineered plagues known as G.U.I.L.T. Unlike normal parasites, G.U.I.L.T. specifically damage the body in a way that ensures the host suffers in agony before dying. If they detect outside interference, such as a doctor trying to save the patient, they speed up the infection to try and kill the host before they can be dealt with.
- Martin Mystery: In "The Beast From Within", Martin is infected and brainwashed by a slug-like alien. Having turned its host into a living incubator for its larvae, the alien spends the episode sadistically stalking the other heroes in the hopes of making them the breeding ground for its offspring.
- Rick and Morty: Played with in "Edge Of Tomorty": here, Rick bumps into an iteration of his dimension inhabited by giant wasp versions of himself and his family — more specifically, parasitoid wasps. Wasp Rick makes it abundantly clear that their way of life involves eating their prey alive and laying their eggs in their eyeballs... and yet, he's easily one of the nicer Ricks in the multiverse, with a much healthier relationship with his family and a happier home life in general. Plus, he's the only Rick encountered in the episode who isn't part of a fascist dictatorship. There's even a heartwarming scene where the Wasp Smiths have dinner together — it's just that dinner just happens to be a screaming caterpillar version of Goldenfold, who is not only fully sentient but also watching in horror as Wasp Morty devours his children. Wasp Rick ends up saving the day by laying his eggs in the newly-incarnated Hologram Rick's eyeballs, killing him before he can kill Rick and Morty.