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Okay, so you've been told that in order to make the Phlebotinum work, or go to this particular place, or what have you, you have to be pure of heart. And then you get there, and there's the Big Bad. What?!? Turns out Pure Is Not Good.
In fact, it's perfectly possible to be Pure Evil.
In Japanese Media, this trope is likely rooted in the philosophical concept of "makoto", which loosely means "pure heart". It basically means a mind free of distractions, unnecessary thoughts, doubts, or fallacies and is mostly used in context of hard work, loyalty and determination. Makoto, while considered a "good" thing, is not limited to good intentions. For example, a villain that showscomplete, fanatical dedication to their cause would be demonstrating "makoto". His dedication is "pure"; his goals, not so much.
This trope is solely for "pure of heart" where purity does not necessarily denote goodness. Any resemblance to being a trope about pure substances is... ahem... purely coincidental.
May make use of Virgin Power as a specific form of purity. See also Ambiguous Innocence, Light Is Not Good, Straight Edge Evil.
Contrast Incorruptible Pure Pureness.
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Anime & Manga
In Dragon Ball Z, it's explained that Vegeta was able to obtain his Super Saiyan form, thought to require a "pure heart", by having a heart of pure evil. One of the difficulties of fighting as a Super Saiyan is the sheer rage that overcomes the Saiyan. This is because their purity of heart is transformed into pure rage. Even for Goku, Super Saiyan was his Super-Powered Evil Side (at first, anyway), which is why he never used the spirit bomb in this form except in a Non-Serial Movie (even then, he had to actually absorb it into his body so it wouldn't backfire on him when he transformed).
In the DBZ Video games, Cell is able to use the Spirit Bomb as his ultimate attack by being pure evil. Incidentally, the Viz translated manga does indeed give this line as a heart of "pure evil".
Kid Buu (Yes, the absolutely insane destroyer of all worlds) is hinted to be pure of heart during the climax of the battle with Goku, since he was able to not only catch the Spirit Bomb and move it back, but was also quite capable of deflecting it back at Goku (During the Vegeta Saga, Goku stated that the Spirit Bomb could be deflected by those pure of heart, and thus could not hurt them). This is probably because of Buu's childlike mentality.
In Bleach, Kaname Tosen says that Wonderweiss Margera likes being near him because pure creatures gravitate toward each other, but that he isn't sure where along the spectrum Margera's purity lies.
A Sailor Moon episode has Minako depressed because the villains haven't tried to steal her pure heart crystal. In order to cheer her up Luna tells her that Usagi had her heart crystal stolen because of her pure love for Mamoru but still has plenty of bad traits like a pure love of eating.
Naruto: Itachi notes that Sasuke is "still pure, and can be colored by anything", and as a result the Big Bad had a fairly easy time manipulating him. Further events indicate this purity likely stemmed from his ignorance of the darker goings-on of his village. The person to first introduce him to the reality could bend his perception of the conflict as they pleased.
YuYu Hakusho has Sensui, who once was extremely pure. If he wasn't so pure as a good guy, he couldn't have become so tainted as a villain. And out of his seven personalities, "Shinobu" (the original) remains very pure and innocent.
Mao from Code Geass is as innocent as a child, and can also read people's minds. If he sees that they've done or thought anything bad, hen automatically assumes that they're bad people. And then he'll use what he knows about them to Mind Rape them. And his victims include the show's resident Messiah and (then) Knight in Shining Armor! It's explained because he received his mind-reading powers when he was only six, and so learned about the bad thoughts everyone gets at some point before he was old enough to understand that you can have bad thoughts or make mistakes and still be a perfectly decent person. To really drive the point home, his signature color is white!
There's an angelic character who is a genuine angel and doesn't have one iota of evil in his heart even when disemboweling his enemies he loves everything. He's on the "evil" side, and since the hero's weapon is only as powerful as the amount of fear of his enemy he's screwed.
Gouda thinks of himself as a pure person and a virgin who has only the best for everyone in mind, but is treacherous bastard who always dresses in black and never had his terribly disfigured face restored, so everyone who meets him will always remember him. He also deliberately designed the Individual Eleven memetic virus to only effect those who were virgins before they were fully cyberized as a further twisting of this trope, since their "purity" is the very thing that makes them vulnerable to becoming his pawns.
Kuze has given up on all kind of idealism and turned to terrorism, but is completely devoted to helping the people suffering in the internment camps and sacrifices everything he has to improve their situation. He also had his cybernetic body custom build with white skin and hair, and a face so perfectly sculpted that it allows for almost no facial expressions or even mouth movement so to not distort it's unfading beauty. He also dresses completely in white when possible evoking at the same time an image of purity and death. He gets rid of the white getup after quitting Individual Eleven and joining the Asian refugees, switching to more practical jeans and leather jacket. Characterwise, he is compared to Che Guevara, Malcolm X and Cassius Clay (the 19th century abolitionist, not the 20th century boxer who changed his name to Muhammed Ali) - a person utterly convinced of the rightness of his cause and with ability to steer others to it with his massive charisma.
In one episode of 009-1, 9-1 meets a young woman who she can only describe as "pure." Later, the woman tries to kill her when 9-1 tries to stop her from meeting up with enemy agents. It turns out that the girl is actually an android made mostly out of very pure gold that the enemy agents were trying to smuggle out of the country.
In InuYasha, Naraku corrupts Hakushin, a Living Buddha who has become bitter over his Heroic Sacrifice, into protecting him by erecting a purifying barrier that destroys demons (except for Naraku's). Kikyo at times is also aligned against the protagonists, and uses her purifying priestess powers to thwart their efforts, since she's a Dark Magical Girl who wants to face Naraku all alone. Ironically (or meaningfully), Kikyou is the one who actually convinces Hakushin to stop serving Naraku, which finally allows him to stop hating people and pass on.
Code:Breaker has the pure, innocent killer child-Sakura. Her pal Ogami shows her that killing is bad and hugging is good, although she kills him anyway (they get better).
In addition, Number 35 (Mariko) is a five year old girl, filled with childish innocence and playfulness. And cruelty.
Mato Kuroi from Black★Rock Shooter is this. She happens to be a genuinely pure girl that has no sorrow or worry, and wants other people to be just like her. In similar case like Light above, she also realizes that people aren't as pure as her and wants to change that. That desire manifests into Black★Rock Shooter and, true to her nature, strives to help others. It's just said Otherself lives in a place where suffering and sorrow are the society, and her method of saving everyone is to kill everyone and everything as a form of Mercy Kill. This makes Black★Rock Shooter extremely homicidal (as the Otherselves are an Emotionless Girl race and only acting on their counterparts' behalf) by going around and killing her own kind in a giant massacre for years without batting an eyelash because Mato is ignorant of such a world and would have never thought that her ideology will result in something like that. When Mato finds out about this, she understandably freaksout.
Becomes borderline Fridge Horror with later episodes when we meet Satsuki's mother Ragyo and find out she sexually molests her daughter and is implied to have been doing so for years, under the pretense of "purifying" her. Yikes.
Kamui Junketsu has a very meaningful name. It is composed of 100% pure Life Fibers and unlike Senketsu, has not been genetically modified in any way. By the standards of his species, Junketsu is indeed the purest can be - a purely hostile extraterrestrial parasite.
The villain of one arc of Noir is Silvana Greone, a Mafia Princess known as the Intoccabile, which translates to "the inviolate" "the untouchable" (here used in the sense of "one you should not touch upon fear of death") or "the pure". Given that she's a villain it's obvious that her type of purity is not the good kind, and terrifies everyone with her Dissonant Serenity and ruthlessness.
In Mobile Suit Gundam: Char's Counterattack the spirit of Phlebotinum Girl Lalah appears in Amuro's dream, saying "Char is pure". At that point, Char is widely viewed as selfish (and/or Messianic) and is trying to render Earth uninhabitable for human life. Amuro's response: to wake up shouting "PURE?!"
Watchmen: Rorsarch has an incorruptible black-and-white world view, that, while admirable in a Crapsack World of Black and Grey Morality, causes him to do some pretty horrific things to people who fall under the "black" part of it. When he knows that the only pragmatic thing to do is to compromise his ideals, he chooses to die instead.
Jei-san of Usagi Yojimbo sees himself as the only person who can eliminate evil. Unfortunately, he is pure evil (with pure white eyes and a creepy voice to boot) and everyone is evil to him, except for his "innocent niece" Kyoko, who is so innocent (or severely traumatized) she isn't afraid of a pile of corpses, only happy that "Uncle is nearby!" Interestingly, Kyoko seems to be the only one (not counting Jei himself or later Inazuma who can safely touch Jei's soul-sucking black blade. Word of God is that Kyoko was supposed to become Jei's new host, but the author felt uncomfortable about it so Inazuma and Jiro were created.
It's established during Final Crisis that the Green Lantern Ring requires a pure willpower to work, and creating a simple arrow is enough to almost take Green Arrow out of combat. It didn't prevent Parallax-possesed Hal Jordan from using it. The other Power Rings appear to work on a similar base, requiring a pure emotion to feed them, not related with straight out good or evil.
In Judge Dredd, when some satanic cultists need to sacrifice a "pure" soul, they choose fascist Anti-Hero Judge Dredd as he's the closest thing to pure in their Crapsack World. Dredd is certainly notjust, but he is utterly incorruptible and the living embodiment of law, order, discipline, duty and sacrifice.
In Trinity War, Pandora's Box plays a major role in the story, and only the strongest or darkest heart can open it. Those that aren't get possessed by it. Superman, the world's greatest hero, was not considered pure enough. It also turns out that no one from this universe is evil enough to open it either. Even Vandal Savage, an immortal Cro-Magnon man who practically invented evil as a human concept, could not open the box. Trying to do it actually brought his repressed guilt to the surface. The Box is opened by the Outsider aka Alfred Pennyworth from Earth-3, the universe that is the birthplace of evil.
In episode 7 of Blade And Soul, Alka runs into Morii again, who hadn't been seen since the first episode and suffering from the loss of her people and home land because they housed the former from the Palam Empire. Morii takes her to see her "mother" the leader of the Boat of Cleansing. However, said house actually turns out to be a Cult, and they nearly kill Alka in a deranged Human Sacrifice.
Like in Death Note canon, Light in A Cure for Love: "At one point he had felt such overpowering empathy that he had to ignore it completely, like flicking a switch. He didn't realize that it was that which had made him cruel."
Film - Animated
In the movie 9, the Fabrication Machine is powered by/contains pure intelligence. Unlike the nine ragdolls, it has no human soul, and therefore no conscience, no sense of right and wrong...Toward the end of the movie, The Scientist explains that it's because of this that the Fabrication Machine is so dangerous: containing only pure intelligence and no human soul, it could be all too easily corrupted.
Film - Live-Action
In Alien, the Xenomorph is said to be "the perfect organism", with "structural perfection" matched only by its hostility, a pure survivor "unclouded by conscience, remorse, or delusions of morality."
In Lord of Illusions, cult leader Nix sermons to his followers that he has been named "the Puritan" by holy decree. They get to find out just how purely evil this guy is when he kills his loyal minions for being mindless slaves not worthy of a being as 'perfect' as him.
In Who Censored Roger Rabbit?, Solomon's lamp is supposed only be destroyable by the pure in heart. In this case, "Purity of Heart" seemed to mean "not acting out of selfish motives". Or maybe, (as the hero suspects) the Purity judge was on a coffee break.
In one of the Magic The Gathering novels, it's mentioned that pure hearts can become pure evil.
In Piers Anthony's Incarnations of Immortality, God is so pure that he cannot comprehend human evil, and so spends all his time contemplating his own greatness. This turns out to be the whole point of the series. God, contemplating himself, isn't doing his job of promoting goodness (or doing anything else at all, for that matter) and needs to be replaced.
In Mark Twain's short novel The Mysterious Stranger, angels are pure - meaning that they have no knowledge of good or evil. The angel in the story causes many characters to suffer horribly, explaining that he can "do no evil" because he is innocent. The angel's name is Satan...
There are several modern takes on Arthurian stories that play up Galahad's faith and purity to the point where he seems distant, inhuman, and utter unsympathetic.
In Zilpha Keatley Snyder's Green-Sky Trilogy, the Kindar race has done very well in purging negative emotions like anger and malice from society, to the point where 2 year olds squabbling over a toy is considered an embarrassing example of poor parenting, and "sorrow" is about as strong as it gets (it's also considered a borderline swear word). Unfortunately for the Kindar, this has led to a very broken Utopia, where the people are dying off.
The Strange Case of Dr. Jekyll and Mr. Hyde. I have observed that when I wore the semblance of Edward Hyde, none could come near to me at first without a visible misgiving of the flesh. This, as I take it, was because all human beings, as we meet them, are commingled out of good and evil: and Edward Hyde, alone in the ranks of mankind, was pure evil.
Henry Jekyll, a man with mostly good and some evil urges, thinks that if he could separate his good and bad urges into separate identities, life would be better, because he would be free of morality and can indulge himself on every pleasurable vice without hypocrisy. When he creates the potion that let him do that, his bad side crosses the Moral Event Horizon and Jekyll’s life is threatened. The only thing that made Jekyll’s safe was his hypocrisy. Mirrored with the impurity of salt of the potion components: the impurities had made the potion work. Without the contamination of the samples, he cannot make the transformation work.
Harry Potter: One could see Dolores Umbridge as indicative of this trope if only as a representative of the concept of purity... in a KKK kinda way.
Umbridge was able to make a Patronus, which requires pure happy thoughts, even while wearing a horcrux, which rendered the heroes unable to create a Patronus. Of course, nobody stated what those happy thoughts had to be about.
And Purebloods in general. Not all pureblood wizards are bad, but those who feel the need to point out that they are pureblood most certainly are. After all, the very notion that those from all-wizarding families are "purer" in blood implies that Muggles and Muggle-born wizards are "dirty" (with the "Mudblood" slur making this clear).
In the Warhammer 40,000Eisenhorn novel Hereticus, Cherubael (a daemon), without being bound as a daemon host, manifests as pure light. The title character notes that its purity is "an abomination". Notably (and perhaps unintentionally hilariously), a minor character who was a priest thinks that Cherubael was a manifestation of the Emperor, and approaches it holding an Aquilanote A two-headed eagle symbol, the equivalent of a crucifix in 40k. and chanting verses and praise. This hurt Cherubael, who ran off briefly, and the priest, confused, gave chase. This gave Eisenhorn enough time to prepare another daemon host to capture Cherubael before it could kill him. (Remember, this was just after Eisenhorn summoned Cherubael to kill a Titan.)
In Leo Frankowski's Conrad Stargard series, the title character reflects on his pastor's words that one with a pure heart has the strength of ten men. He realizes that it could be pure anything; pure love or pure hate, pure greed or even pure evil.
One of the most common Kurt Vonnegut quotes comes from the protagonist in Mother Night, and it could be this trope's page quote: "Say what you will about the sweet miracle of unquestioning faith, I consider a capacity for it terrifying and absolutely vile!"
In His Dark Materials specifically the last installment The Amber Spyglass a priest is sent after our hapless heroes; this man's special because among his peers he is seen as inexplicably pure - he has spent a long time praying and being absolved so that he could 'make room' for the sin of committing murder.
In The Hunchback of Notre Dame, the Hunchback and Count Frollo are similar to Light Yagami in that they are so pure (they lived their whole lives in the cathedral) that when sin (in the form of Esmeralda) enters their lives, they have no idea how to deal with it. Frollo becomes obsessed with her yet hates her for giving him impure thoughts.
In the Lord of the Rings series, as a being of pure hunger, Shelob cannot be tempted by The One Ring's promises of power.
Tom Bombadil is equally incorruptible by the Ring, and although he is benevolent to the hobbits (and to pretty much everyone who isn't working for the Dark Lord), his incorruptibility makes him too fickle to be a reliable caretaker for the One Ring.
In Buffy the Vampire Slayer, Spike and Drusilla unleash the Judge, a demon sent to destroy everyone that isn't pure evil. It's a pretty demanding requirement - Spike and Drusilla themselves were vulnerable because of their love for each other, and their minion Dalton died because of his love for learning. The only one confirmed to be immune was the newly released Angelus.
The Judge: There's no humanity in him.
Angelus: I couldn't have said it better myself.
The demonic types that make the most of being pure are the Scourge (who want to exterminate both humanity and impure demons, though their own purity is dubious by other canon) and the Old Ones, the monstrous original demons.
Xander also attempts a love spell on Cordelia so he can dump her, and is told that the emotion has to be pure. He replies that he intends pure revenge. The spell works, although not the way he wants it to.
In the Biblical tale of Adam and Eve, before eating from the Tree of Knowledge, mankind was "innocent" in the same way as animals, which in context means that they were ignorant of any concept of morality. Essentially, they were sociopathic.
Lawful and Chaotic Neutral also fall here, albeit in a downplayed manner — at their extremes, they are pure Law or Chaos, and hence Pure but Not Good or Evil.
The Architects of the Flesh from the Feng Shui RPG seem to take this idea to heart in many ways. In their world intra-racial (within one's own race) relationships are socially unacceptable, their primary technology (arcanowave) is demoniacally imbued cybernetics and, when their program to enhance apes with cybernetic implants turned against them, they took up capturing monsters and replacing some of their innate abilities with arcanowave technologies (possibly to hide the fact that most of the human users had turned into monsters as well).
Exalted: The Primordials are so pure, they are the embodiment of their concept. So Malfeas is pure RRRAAAGGGEEE, She Who Lives In Her Name is pure Order, and the Ebon Dragon is pure Betrayal (let that last one sink in for a bit). This turned out to be their downfall, because they absolutely can't understand why anyone would think differently than how they think.
While the Primordials may not understand why people think differently, they do comprehend that people do think differently. In the mind of She Who Lives In Her Name, the Primordials' decision to allow free will in the world they created was a bad idea, especially since it got her and hers thrown into Hell. Should she ever be released, she fully intends to put that right.
Possibly the most tragic part of the Primordials' example is that they were the purest essence of their concept including the good parts... until the Exalts killed parts of them and forced surrender oaths on the rest that pretty much eliminated the good parts. Malfeas was once the purest expression of rulership, including the guarding steward as well as the tyrannical lord. When the Exalts were done with him he became the deposed tyrant, who rages because he (in his mind) should be king but cannot be. The Ebon Dragon was always a pure asshole, though.
The Chaos Gods of the Warhammer universe represent different emotions distilled to their purest form, spelling trouble for everyone.
Also, while normal creatures exposed to Chaos become Corrupted, entities native to Chaos do not, largely because of this trope.
Nobilis: Deceivers whose lifepath includes Water Lily, the Key of Something Spiritual, are pure and innocent Omnicidal Maniacs.
Uriel was the Archangel of purity in In Nomine. He led a crusade of genocide against the world's pagan gods and myths, driving many of them into the arms of hell.
The Xel'naga of Starcraft proved this to be true in both of the races they personally nurtured. The Protoss were molded in pursuit of "Purity of Form" and the Zerg in an attempt to maintain a "Purity of Essence". Both ultimately turned on their creators.
In Star Fox Adventures, an Earthwalker tells Krystal that only the pure of heart can take the tests. Later, a Krazoa tells Fox that only the pure of heart can enter the shrines. So we take and pass the tests, and go to the last shrine, and...cue Scales.
In Luminous Arc 2 Elicia, the Big Bad is actually a Messianic Archetype who saved many children's lives, but the grief that she cannot save everyone cause her to go to the dark side and tap into forbidden magic.
Evil and Pure characters will resemble vampires with pale skin and red eyes. Their character type is the Fanatic.
Specifically, townspeople will be afraid but attracted to you if you are evil/pure, and do fun things such as spout off a line about how 'you should just die', then immediately follow it up by asking you to marry them. In contrast, a good/corrupt character is liked by everyone, but has a harder time causing townsfolk to fall in love with them or take them seriously.
The meter for Purity/Corruption thereof could be seen as a sliding scale of Lawful/Chaotic at the same time.
Roa of Tsukihime was an incredibly pure person in life. Also a good person, from what we hear during Ciel's route. Unfortunately, this gave him absolutely no understanding of his emotions so, lacking understanding of his love towards Arcueid, he turned into a Big Bad whose purpose was to continually torment her because he didn't know how else to express said feelings.
In Fate/stay night, Gilgamesh reveals that the Grail spilled on him in the last grail war. The Grail contains the incarnation of all human evils in the world, and its contents tend to hideously corrupt and/or dissolve whatever it touches — But it failed to affect him at all because he was so pure. Because he's simply thatpurely self-centred.
Gilgamesh: "All the evils in the world"? Bring three times as much if you want to stain me!
When the scene is revealed in its entirety in Fate/Zero it turns out Gilgamesh's ego is so great and his existence so absolute, the Made of Evil entity that exists inside the Grail can't shake his resolve. Gilgamesh is basically able to tell if off by taking responsibility for all of humanity's sins, an act he is able to do because of his titanic ego.
Played for laughs in Disgaea, shortly after Jennifer meets "Prince" Laharl for the first time:
Jennifer: I believe you because your eyes are pure. I don't know if it's pure good or pure evil, though. (chuckle)
In Sengoku Rance, Natori eventually reveals to Rance that the reason he kept losing the Miko lottery (you get to have sex with a miko, nearly everyone wins) is because he has no negative emotions for them to absorb. No dishonesty, jealousy or hate. Yep, Rance is very open about his desire to have sex with absolutely anything female and cute whether they like it or not.
Wendy from Rule of Roseis the sweetest little girl that you could think of, and her love and devotion for Jennifer is absolute. Pity that she expects just as much in return, and won't accept tiniest compromises.
After the player learns N's backstory from one of the "goddesses" at the end of Pokémon Black and White, she even lampshades this. N's innocence, purity and good will are what makes him dangerous in the wrong hands.
In What Did I Do To Deserve This My Lord, Pure is good, but it's no good for you. Monsters that are under a lot of stress may tend to give birth to "Pure" versions of monsters which are weaker and do not reproduce and are so nice that they're terrible at fighting the heroes. The third game also introduces holy water, which can purify your monsters into the lame versions.
Ishida Mitsunari from Sengoku Basara 3 is described as 'disconcertingly pure' during his brief cameo in the game's anime. This also implied in Ieyasu's blue path. His personality in-game can charitably be described as 'focused' and 'uncompromising'.
Maiden Astraea from Demon's Souls, The church dislikes her and says her kindness was too pure, and it lead her to evil. But this is not the case: she took a demon's soul and lives in the swamp, where she cares for the undead aborted fetuses, you read that right, and tries to live in peace. She only attacks in self defense. She's also the least hypocritical character in the game.
Skullgirls has a tentative example of this in Cerebella's individual storymode. It's noted that Cerebella has a pure heart, but that doesn't stop her from killing Ms. Fortune by crushing her into a new Life Gem. Sadly, Cerebella seems to realize her folly only after she presented her mobster father figure Vitale with a shiny new Life Gem made of compressed cat gore.
In Touhou: Subterranean Animism, the description for Parsee's "Grandfather Cherry Blossoms" spell cards notes that neither Reimu nor Marisa is pure-hearted enough to make flowers bloom from the ashes, so they must be reacting to Parsee's own pure envy.
In the Silent Sinner in Blue manga and its associated material it's revealed that Impurity is the cause of death and death is the source of Impurity. It's also revealed that the reason behind the Lunarians' vast lifespans is that they, and the Moon, are Pure due to never having been touched by death... But for all their Purity the Lunarians are also generally elitist jerks and simply being Impure is a crime punishable by exile in their society. Or in other words:
Watatsuki no Toyohime: Simply living and dying here on Earth is sin enough. Your punishment will be decided after we bring you to the moon... and the punishment for the creatures here on Earth... is to spend their entire lives creeping about the Earth, and finally die.
OFF gets a nasty example of this. Try going to one of the zones after you beat it sometime.
Even worse, purifying the world means personally killing the Batter's wife and Hugo, who technically isn't their biological child (in fact, Hugo created both of them) but either way the Batter still personally kills a small child.
Sluggy Freelance played around with this in the That Which Redeems arcs. The inhabitants of the Dimension of Lame are all incredibly innocent and pure, to the point of being annoying. But as a whole, most of them weren't exactly good. They were perfectly willing to let demons from the Dimension of Pain enslave them, and tried to hand Torg over to them multiple times on the flimsiest promises of lesser suffering from the demons.
And their Riff ended up being the sole demon in the world after victory, fitting due to his causing all of the problems by kidnapping Torgs from various dimensions after accidentally blowing up his own.
In Ansem Retort Axel manages to achieve "Reverse Nirvana" because his heart is pure chaotic evil.
It's mentioned that the only other person to achieve Reverse Nirvana is Black Mage.
Though they're more Well Intentioned Extremists than anything, anytime the Kyorl'solenurn in Drowtales invoke the word "purity" it's generally a good idea to run since they're probably about to perform a "purge", IE kill a bunch of people (who may or may not deserve it, it varies).
In Sailor Nothing, the Yamiko created from a person has a power and evil level proportional to their host's purity.
Or rather, how much they've repressed their impurity; nobody in this 'verse is pure in standard sense. Because good people are better at resisting bad urges, there's more repressed bad urges to make into a yamiko. A bad person doesn't hold much back, leaving little to make a yamiko with.
Dragon Ball Z Abridged has the "Vegeta going Super Saiyan" example above, except instead of pure evil, he claims it's because he's "pure unadulterated badass."
Avatar: The Last Airbender: To bend lightning, one must have no internal conflicts whether emotional or idealistic. Purity of focus is required. You can do that by being calm, cool and collected like the well-adjusted Iroh, being an emotionally repressed psychopath like Azula or being ruthless and supremely arrogant like Ozai. Anti-Hero Zuko, however, can't manage it because he's too conflicted and generally emotional.
Deconstructed in Young Justice: Harm became pure evil by murdering his own sister, the only person he ever loved, so he could wield the Sword of Beowulf, which only the pure of heart can draw. The Sword rejects him when he feels a shred of remorse after seeing his sister's ghost, becoming impure. The point is, of course, that while Pure Is Not Good, pure is also not easy.