"Our firm has always been here. In one form or another. The Inquisition. The Khmer Rouge. We were there when the very first cave man clubbed his neighbor. See, we're in the hearts and minds of every single living being. And that — friend — is what's making things so difficult for you. See, the world doesn't work in spite of evil, Angel. It works with us. It works because of us."
— Holland Manners explaining Wolfram & Hart, Angel
As a Death Trope, all Spoilers will be unmarked ahead. Beware.
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In the anime adaptation of Chrono Crusade , the Big Bad, Aion, uses this trait to come Back from the Dead after the heroes defeat him — he dies, but there's so much hatred in the area that he instantly rises. The anime ending also suggests that he might be The Heartless.
The Great Leviathan's shadow form is said to be fueled by darkness in the hearts of people. So long as there are those who succumb to their darkness, the Great Leviathan shall never truly die. This is probably a subversion, since the Pharoah responds to this by saying that's crock and that it was actually created by the Orichalcos itself, and then appears to prove that by seemingly vanquishing the Leviathan with his power forever.
This is done a second time in GX with Darkness/Nightshroud.
This trope still applies, because one of the biggest themes in Madoka Magica is that curses will always arise; if one curse is destroyed, that just clears the way for another, possibly more powerful one. Happiness must always be balanced out by despair. It's the reason that, even after Madoka's wish, Puella Magis still have to fight demons instead of witches; misfortune and despair still exist in the world and are going to find some sort of magical personification for them to fight.
In Hellboy: "The Baba Yaga". It's said that she cannot die as long as Mother Russia endures. That said, Hellboy shooting her in the eye effectively banishes her to non-physical realms.
After the Fantastic Four villain Blastaar gets electrocuted in a Silver Age story, Cyclops explains "Blastaar's basic energy was - evil! Pure, unadulterated hate! And wherever men live with hate in their hearts - Blastaar lives there too!" This is not officially one of Blastaar's powers, but since he came back to life without explanation shortly afterwards, Cyke presumably knew what he was talking about.
The Shadow King fits this trope to the letter, especially following the retcon that he's an ancient demonic being, and not the psychic remains of an evil mutant.
At one point CaptainNazi claimed he was a incarnation of Nazism, and as long as someone believed in it, he could never truly die. This is an upgrade from a cut-price evil version of Captain America, as was his previous origin.
This is outright said to be one of Mephisto's abilities.
As long as there is life, the universe will never be without fear!
In the movie Legend, Darkness (played by Tim Curry) mutters a textbook speech about "being part" of all of the heroes before dying. His laughing ominous face in the last shot of the movie while the heroes are frolicking in the sunshine indicates that he is correct.
In Time Bandits by Terry Gilliam, one piece of the aptly-named Evil, the Made of Evil antagonist, escapes the cleanup crew to afflict the world again. The Supreme Being informs the kid that he must "carry on the fight". The first thing it does is manifest as a roast in his parents' microwave, killing them.
Star Wars: the Sith are an interesting example because they are a group rather than an individual being - no matter how many times the Sith Order is exterminated, it will always rise again because of the seductive appeal of its teachings. This is really obvious in the Expanded Universe, but you can see it a bit in the movies as well.
The novelization of Episode III includes an interesting bit of poetry, split between the major sections, about evil and darkness - how it is powerful, and seductive, and can never be defeated because it is everywhere: "The brightest light casts the darkest shadow." At the end, the piece also points out that "it has a weakness. A single candle flame can hold it back."
Love is more than a candle. Love can ignite the stars.
Jolee Bindo and Kreia both lampshade this in Knights of the Old Republic by pointing out that no matter what happens, the Sith and Jedi will always return from the brink of extinction and fight each other into near-oblivion.
The Darth Bane trilogy examines things from the Sith's perspective. The third book strongly implies that the galaxy needs evil monsters like the Sith in it because otherwise the Jedi Order becomes stagnant and corrupt without having any Sith to fight. The Jedi lose their sense of right and wrong, which results in them doing what is politically expedient and not disturbing any peace as opposed to doing what is right. Everyone else takes on a "It's all about me" mentality, which results in them doing what benefits them as opposed to doing what is right. The Sith bring about change, because nothing holds them back. They take the "It's all about me" mentality to its logical conclusion. They are the ones who exist to show everyone that they need standards, and that there has to be a sense of right and wrong that applies to as many people as possible and not just a different sense of right and wrong for each person.
Batman Begins: Ra's Al Ghul cites civilization's cyclic decay into criminality as the reason for the League of Shadows' existence.
Dragonheart is one of the rare cases that combines this with As Long As There Is One Man. When Bowen protests that Draco's death is unnecessary since their allies have already taken the castle from Einon, Draco responds that they will never win as long as Einon's evil endures.
It seems that as long as fear (especially of him) exists, so will Freddy Krueger. One of the short stories in a collection rolled with this, having Freddy claim that while his primary fuel is fear, other negative emotions (like hatred and resentment) can work just as well. As long as people keep feeling those, he'll never be permanently stopped.
Monsters in Girl Vs Monster are fueled by fear and will continue to exist as long as someone, somewhere feels fear. They can be sealed away and during that time the person will not feel fear.
The Silmarillion: Morgoth. "Yet the lies that Melkor, the mighty and the accursed, Morgoth Bauglir, the Power of Terror and of Hate, sowed in the hearts of Elves and Men are a seed that does not die and cannot be destroyed; and ever and anon it sprouts anew, and will bear dark fruit even unto the latest days."
This is a running theme in J. R. R. Tolkien's books. As long as darkness persists in the hearts of men, evil "will ever take another shape and grow again." Yet only Men with their Gift of Freedom and power to write their own destinies have any hope of finally redeeming Middle Earth.
One of the characters in Cryptonomicon uses this as a time scale for how long he wants his secrets kept.
Death in Good Omens makes this sort of a speech after the other three Horsemen of the Apocalypse have been defeated. He claims to be necessary for reality. War, Famine and Pollution (Pestilence quit around the time penicillin was discovered) are all creations of human beings, and as such will come back soon, but he's been around long before any of them, and can't be destroyed at all.
The Gloamglozer: "So long as the strong pick on the weak, so long as fear is valued above tenderness, so long as hatred, envy, and mistrust divide the various creatures of the Edge, then I am indestructible!"
This was the point of a short story where people literally build a gigantic bonfire in the Great Plains and burn everything that they hate or that causes them problems. Satan is watching from the sidelines. The Protagonist asks him why he's chuckling — surely this marks a new milestone for mankind — and Satan answers: "Can you burn your own heart?"
The Serpents from The Death Gate Cycle are like this- they literally are evil, given shape and form by magic gone mad, and will exist for as long as mortals do. Creepily, whenever someone asks who created them (the series' universe was built by a race of Physical Gods who most certainly didn't intend to make the Serpents, so this is a legitimate question), the response is always a whispered "You did". Thankfully, they have good counterparts who are just as eternal.
Word Of God has already confirmed that the Dark One won't be destroyed at the end of The Wheel of Time - indeed, he can't be destroyed. The last book proves that the Dark One, personification of evil that it is/does provide something essential for human beings to be human. Rand sees a vision of a world where he did kill it, and it's a world where everyone is sappily happy all the time because they don't have the option of being anything else. Though Rand could kill the Dark One, he ends up sparing (and resealing it) instead as a result.
Randall Flagg in The Dark Tower and The Stand. Lampshaded at the end of The Stand when he washes up on the tropical island to, as the chapter title says, close the circle.
Webster: Well, I never heard the dev— of you claiming American citizenship.
Mr. Scratch: And who with better right? When the first wrong was done to the first Indian, I was there. When the first slaver put out for the Congo, I stood on the deck. Am I not spoken of, still, in every church in New England? 'Tis true, the North claims me for a Southerner, and the South for a Northerner, but I am neither. To tell the truth, Mr. Webster, though I don't like to boast of it, my name is older in the country than yours.
The First is the first evil to ever be, and will live as long as evil itself does.
Likewise, Wolfram & Hart in Angel, like the quote listed above. According to Illyria, they existed in her time as well but were weak then, and as such might be technically destructible entities that simply draw power from humanity's evil rather than being created by it. Even if the "Senior Partners" were killed, it's likely that Wolfram & Hart as an organization would continue to operate.
"I WILL NEVER DIE!!! THE THOUGHT OF ME IS FOREVER, IN THE BLEEDING HEARTS OF MEN, IN THEIR VANITY AND OBSESSIONS AND LUST!!! NOTHING SHALL EVER DESTROY ME!!! NOTHING!!!"
shortly after this, he gets tossed into a black hole by Rose.
In the tokusatsu series Garo, the monsters are "horrors", creatures born from the darkness in men's souls. Since there will always be darkness in Man there will always be horrors, and therefore there must always be Makai Knights to fight them.
The Twilight Zone episode "He's Alive". In it, a young American fascist is guided by the ghost of Adolf Hitler. As part of the the usual end-of-episode narration, Rod Serling states that he will continue to exist as long as hate exists.
Ultraman Ace villain Yapool in modern incarnations (such as Ultraman Mebius) has this. He's been stopped or killed 3 times over the series. How long it takes him to recover each time he's been killed varies from a few months to almost 20 years.
Chaotica, villain of the Captain Proton holodeck "show" in Star Trek: Voyager, claimed this when he was defeated and killed. The character was basically a love letter to over-the-top villain tropes, so this isn't surprising.
In Twin Peaks, Albert invokes this trope in a monologue he gives shortly after the initial defeat of the series' Big Bad BOB.
Variation: the Backstreet Boy's song "Everybody (Backstreet's Back)" claims "As long as there be music we'll be coming back again."
Naturally, Voltaire's song "When You're Evil":
While there's children to make sad,
While there's candy to be had,
While there's pockets left to pick,
While there's grannies left to trip down the stairs,
Whenever the dark is near, I will return from the grave to haunt you...
The various Warp-entities in Warhammer 40000 are formed from the thoughts and desires and emotions of living creatures in the Materium, or the "real" world.
Since these daemons are formed of pure thought and emotion, they cannot be truly killed, and instead only banished back to the warp. There are a few exceptions to this: a powerful enough mystical attack capable of shredding them into their component bits of soulstuff*
Gregor Eisenhorn manages this against a daemonhost in the second volume of his trilogy.
or the anti-psyker abilities of a powerful enough souless Blank which can deal permanent harm to the entirely warp-based form of a daemon.
The Chaos Gods are extremely powerful warp-entities who feed off of the various emotions of humans and other living beings — but it's not just the negative ones. Hope powers Tzeentch, god of change; bravery for Khorne, god of war and violence; love for Slaanesh, god of pleasure; and endurance for Nurgle, god of disease and pestilence. Direct Worship provides a pretty large boost in power but is not necessary- The Warp was far less turbulent during the Horus Heresy when the majority of humanity genuinely scared the Chaos gods. These beings are so powerful they can't be destroyed by anything less than another Chaos God. This comes at the price of not being able to manifest themselves in the material world and needing to send their greater demon avatars and lesser demons as well as using worshipers to spread their will.
the Necrons (Undead Robots in Space) have a plan here. If the existence of the chaos gods is perpetuated by all of the hopes, dreams, desires, and everything that makes sentient life sentient, the logical method for the removal of the threat of Chaos is to simply exterminate all life everywhere. For obvious reasons, the other races aren't too keen on this plan, but at least they have one. Then again, they also serve the C'tan (Star Gods), who want to eat sentient life or their souls or their Life Energy which can't happen if all life everywhere is exterminated. It's complicated.
Warhammer. Some of The Undead (actual undead this time) also have a plan which is essentially the reverse of the C'tan's idea. They intend to convert all mortals into undead, thus starving Chaos of emotions to feed upon.
The Rakshasa demons in the Eberron campaign setting are like this. It has been canonically stated that as soon as one is killed, a new Rakshasa springs into existence somewhere else.
Exalted: As long as someone knows one of their Charm, a Primordial can never be annihilated for a Primordial is the totality of their Charms. In fact, it's possible that Primordials can't cease to exist unless Creation itself cease to exist, as the Neverborn can attest.
In the survival horror game Alan Wake, the main villain Barbara Jagger says right before you destroy it by sticking a lightswitch into its chest says "I will find a new face to wear, new bones to set me free!" At the end it shows that the dark presence DID find a new face to wear; Agent Nightingale's face.
In Splatterhouse 3, if you get a bad ending, the Terror Mask mockingly informs you, "I feed on human suffering. So long as humans feel pain, I will exist!" Get the good ending and it doesn't get to mock you before dying.
Dracula constantly gives this speech in every game he appears in, however, he implies he's tired of his role in Castlevania Symphony Of The Night. This weariness is what leads to his (as of yet undepicted) final defeat in 1999.
The Balance Between Good and Evil is having a hard time finding someone as evil as he was, and tries to recruit his Reincarnation in the Sorrow games. If Soma rejects his previous life, then Alucard muses that while an Evil King may be inevitable, free will means that it doesn't have to be one particular person.
Because of that, Sora keeps getting roped into playing "Whack-a-Mole" with all the wanna-be candidates for Dracula's old position. Since he's prime candidate, they all want to kill him to prove how bad they are.
In Order Of Ecclesia, Barlowe's true goal is to use Dominus in order to bring back Dracula, since he "reasons" that because he keeps coming back, that is Mankind's true dream.
Defied in the flash game Malapa's Challenge, where two different villains start to give this speech... then are banished from our reality anyway. Apparently, having a direct line to cosmic forces won't stop someone from punching you out if they try hard enough.
The speeches were slightly more inspired by Humans Are The Real Monsters than As Long as There Is Evil; since they view humans primarily as evil in nature, the dark gods feel that they will always have a leg up when dealing with mankind. (As they are immortal, there will always be evil, but that also means that there will always be good as well.) It was also to try to instill into the main character that he was alone in the fight, since there were hordes of monsters and a bunch of bosses, yet not another light god would bother to fight along side him. Once more, this was a subversion as well, since there are two instances of light gods intervening after boss fights, one actually interrupting the "You are completely alone" speech; even Zeus has to give him a speech about how he is not alone at the end of the game. Needless to say, many things that the dark gods told Malapa were Mind Screws that were going to be explored in the latter games, as well as why some of the gods were acting very out of character. (Too bad they never happened...)
This trope seems to be a recurring theme of the Persona series —
Nyx from Persona 3, is the Anthropomorphic Personification of death and therefore cannot die — the heroes are forced to seal it away again to keep it from destroying the world, knowing full well it may return later if any more meddling Straw Nihilists come along and try to re-summon it. In The Answer, the new chapter for Persona 3: FES, it's revealed that the seal isn't to protect mankind from Nyx, but to protect Nyx from the desire of death that called Nyx down (Erebus the death monster) — she truly doesn't wish to destroy the world, and is happy when the heroes defeated her in The Journey (the original storyline) but as long as humanity desires death, Erebus will exist forever.
The Nyx example happens as in the original game, an optional Snow Queen sidequest that has the "Night Queen" try to use people in order to set her free so she can freeze everyone alive in statues and live under an eternal night. A variant of The Fall was in the very first game. THAT'S planning.
Ameno-Sagiri and Izanami in Persona 4... but played with for Izanami, as she promises to leave humanity alone, having completed her 'experiment'; she just can't promise that humanity won't try to deceive itself again, which could re-summon her.
Hearkening back to the Zeromus speech, we have Odio's speech at the end of Live A Live. As the heroes kill off his various incarnations throughout the time stream, he asks why he can never win, as his reason for becoming Odio to begin with was that he had lost everything, and wanted stupid humans to know his pain and see the error in waiting for heroes to fix everything. The chapter's main character informs him that he lost because he gave up his humanity and hated humans, even though he was originally a human himself. Coming to his senses, he delivers a stern warning to the main characters before dying.
Odio: "As long as there is hatred, anyone can become a demon."
He also gives a longer speech before the final Boss Rush:
Odio: "Even if I'm beaten... I'll keep on living... You must know... The meaning of Odio! It is.... From ancient times.... Until the distant future! In eras of peace.... And eras of war! In all places! And all eras! The impetus of all conflict! The eternal sentiment that will continue as long as humans exist... Its name is.... Hatred.... It is Odio!"
Deathevan, the Big Bad of Breath Of Fire II, feeds off of the darkness in human souls. In the end, it is explicitly stated that this means Deathevan can never truly be killed, and so protagonist Ryu decides to transform into a dragon and seal the gate to the underworld in the hopes of thwarting his inevitable emergence. Depending on whether or not certain conditions are fulfilled, Ryu will either do so, much to everyone's dismay, or his father Ganer will crash the flying Township down on the gate, sealing it permanently, after which he delivers a speech telling Ryu that if he can eliminate hate and despair from the world, Deathevan will never be able to return.
Bacterian: "I was born out of the greediness of mankind. While men exist, so will I!"
In the first game in the Fire Emblem series, the dragon Medeus claims he gains strength from the evil in mankind when Marth defeats him.
In Fire Emblem: Genealogy of the Holy War, Julius, aka the human vessel of Earth Dragon Loptyr, spouts basically the same speech to Seliph if he's the one who defeats him ("As long as greed exists in men's hearts, I will return!"). It's no coincidence, as per All There in the Manual, Jugdral and Archanea share the same universe, and Galle, Julius's ancestor, during his long travels around the world, happened to visit Archanea and formed a pact with Loptyr, an evil member of the Dragon Tribe, by drinking his blood.
Wizeman: I am the creator of Nightmare... As long as darkness exists in the hearts of humans, then I, and the nightmares, will never be destroyed...
In the special ending (where he doesn't say this), it takes a Heroic Sacrifice on the part of Nights to finish him off. As it's implied that Nights is still alive, it's possible that Wizeman is Not Quite Dead.
In City of Villains, spirits must be anchored to a person to remain in the physical world. Except for Ghost Widow, who is bound not to a single person, but the very concept of the Arachnos organization, and will continue to exist as a villain so long as there's even a single person identifying themselves as an Arachnos.
They may have given a separate trope its name, but The Heartless count for this trope too. Yen Sid says it himself: so long as darkness exists in people's hearts, the Heartless will continue to spawn. They've been reduced to more of a nuisance due to the events of the first game but they still have the potential to rise.
Inverted in the first game; Sora states that even the greatest Darkness had to contain a spark of light that could never be extinguished. So, As Long as There Is Evil, there is Good.
Kingdom Hearts Chainof Memories: At the end of Riku's story Ansem states that he can return using his darkness that he gave to Riku. In the final battle in "Kingdomhearts358/2Days" he's proven right.
YHWH, trademark Big Bad of the Shin Megami Tensei series, will exist in his current form for eternity as long as at least one human being believes in him as a true god. Since the god and goddesses are supposed to reincarnate after a while, this is problematic for the universe at large. Word Of God is that it's not entirely humanity's fault; YHVH being a warped despot is a symptom of something seriously wrong with the fabric of the universe.
Gods in Sacrifice cannot truly die — their divine essence will reform them sooner or later into a similar role, due to the fact that, being gods, they're intrinsically bound to the land and its people. Their current personality and name, however, disappear with the current incarnation — what comes back isn't precisely the entity that disappeared.
The Super Robot WarsBig Bad Dark Brain is pretty much stated to be immortal and having unlimited power until all negative emotions in all of existence are removed. Yeah, good luck with that.
"If evil disappears, so will the hero. But if evil lives..."
Gnarl, of Overlord, seems to believe that no matter what happens, there will always be someone to take up the mantle of Evil Overlord, thus restoring evil to the world. The games support his point. However, since 'Good' is worse then 'Evil' in this world, due to it being a Crapsack World, this isn't as bad as other examples.
In the Mata Nui Online Game, according to him at least, Makuta is the essence of destruction, and that he is inside anyone, no matter how innocent, as long as they have the capability of destruction. Oddly enough, he never says evil; he instead says that he is destruction.
The Grey Wardens managed to find a way around this during the First Blight in the backstory of Dragon Age: Origins, but if you don't have a Grey Warden handy then you can never actually kill an Archdemon.
Knight-Commander Meredith uses this line as justification for her stonewalling the election of a new viscount at the opening scene in Act III of Dragon Age II.
Julius from Sword of Mana has a variant which he claims upon death. As long as there is Mana power in the world, he (or rather, Vandole) will always be reborn. The thing is, he destroyed the Mana Tree in order to go One-Winged Angel, so he actually won't be reborn unless the world is so dependent on Mana that the heroes are willing to sacrifice the heroine to make a new one. It is, and they are. What's more, Julius himself doesn't believe the world needs Mana, which might just change the meaning of his last words to "I won't come back unless you humans are stupid enough to give me a reason to return", making it a more accurate example of this trope.
In The Elder Scrolls series the Daedra, most of whom are evil or amoral enough to be perceived as such, are almost impossible to truly destroy. They are manifestations of the primal forces of reality so even if their avatar was somehow destroyed, a new avatar would form to take their place. The best anyone can do is shatter their link to the mortal realm and banish them back into Oblivion.
In the LittleBigPlanet 2 story mode, right before you fight the final battle against the Negativitron, he states one of these.
Negativitron: "You can never truly defeat me! I am in all of you... I AM all of you!"
In Adventure Quest Worlds, the main boss of the Fear Chaser event is a gigantic entity called FEAR, who, as his name suggests, is a personification of fear. The reason he exists is because fear is a part of life. From the dialogues gathered in the cutscenes before and after defeating him, he will always exist because most people need fear for said reason.
FEAR: From the first moment the first creature appeared, it was afraid. Afraid of the unknown, afraid of starving... afraid of dying. Life IS fear. Fear can swallow even the bravest of heroes. Even the mighty Fear Chaser knows she can never truly destroy me.
The Dark Genie in Dark Cloud. It flat out tells you it has no physical form, and that its defeat means nothing, as it will always be brought into existence so long as there is hate. Because it exists outside of time, it will have always been going to be created.
The first Myth game invokes this to make the ending less bittersweet. Over the course of the game, it is revealed that the world's history is cyclical. After a thousand years, a malevolent, transient Divinity called The Leveler will arise and wage war on all civilization, eventually destroying it and plunging the world into a Dark Age for a thousand years. At the end of this period, a hero will arise, defeat The Leveler, and usher in a Golden Age that lasts for another thousand years—at which point the Leveler returns to start the cycle anew. The reason this makes the first game's ending bittersweet is that the mortal Legion is destroyed and their cities have fallen, but their hero, Alric, breaks the cycle by casting the Leveler's vessel into the Great Devoid.
Myth II affirms that the cycle has been broken, being set roughly 60 years later and showing that Soulblighter has been left with a world that has recovered enough to be worth conquering. He doesn't win, and the final narration contemptuously declared that this is because he was not the Leveler. This makes Myth III, a prequel, that much more bittersweet.
The Legend of Zelda: Skyward Sword reveals why the series' Big Bad, Ganon/Ganondorf always appears. After being defeated by Link, Demise places a curse on Link and Zelda that forces their descendants to fighting Demise's hatred in reincarnated form for eternity. No matter how many times Link and Zelda defeat Ganon/Ganondorf, he will always come back eventually.
Diablo: The series has fun with this trope. All three games end up revealing that Angels and Demons have fought each other since the beginning of time. However, things changed when a number of individuals on both sides got tired of the conflict and created Sanctuary and Nephalem, and humans are descended from them. Then you have both sides trying to figure out what to do with this third group, since they have the potential to be more powerful than both sides, actually possess free will, and they are Immune To Fate. In fact, Diablo III shows that demons can come back from being killed, even if it takes 20 years for them to do so. Of course, when a Nephalem kills Diablo at the end, Tyrael is quick to say that evil has been defeated forever and the eternal conflict is finally over. The Nephalem points out "Don't be so sure. True evil never dies," and Tyrael responds "Time will tell." Yes, time will tell....
Blizzard's other franchise, World of Warcraft, introduced the Sha. Shadowy beings native to Pandaria, they feed on negative emotions. Their nature makes it impossible to kill the Sha; their physical form can be destroyed but the Sha itself will eventually reform if it finds a source of negative emotion.
Ninja Boss: Fighter, you have one this battle but there is one rule for the univarse and that rule is that there is always more ninja!
Fighter: Oh yeah sneaky naninja boss? Well I happen to know another rule of the infinite univarse! And that rule is that there is always more swords to kill ninjas with!! I don't know what that means but it is teh truth!
Defied In Oglaf the worshipers of fun claim that even with their god dead they will continue to exist as long as there is injustice and suffering for them to point and laugh at. However, Greir then burns down their temple, which puts a stop to them.
In Underling, Satan. Though it's really more of a weakness.
The Blood Red King, the Anthropomorphic Personification of Anger, Cruelty, Violence, and Fear in the Global Guardians PBEM Universe, has been killed numerous times since he first appeared in the late 1970s. But as long as there are people being angry or cruel, causing violence or causing fear, he always springs back up shortly thereafter, fresh as a daisy.
This is how Cancer survives at the end of When the Puppy gets Lucky. As long as there is cigarette smokers, Chimio will be unable to finally kill it.
Unicron in Transformers. It's outright stated in Transformers Armada that he'll exist as long as hate exists (and the Autobot-Decepticon war is mighty good eatin.') he doesn't need to eat planets; that's just because of a personal vendetta against existence itself. It appears on the surface that he's doing all that eating out of hunger, but no. He just wants to be really alone, and has the planet-sized balls to do something about it.
Xiaolin Showdown, throughout the series and especially at the ending. Master Fung always implies that evil is never defeated, but merely changes its path.
Spirit: Yes, but in a sense it matters little. There will always be evil. And evil men.
Scarlet: Yeah. Good thing there'll always be us Joes.''
Invoked in X-Men. Beast ponders whether or not Apocalypse, as a personification of evil, can truly be destroyed or if a new evil will simply take Apocalypse's place. Cable replies that he doesn't care.
In the Justice League episode "Hawk and Dove", Ares declares "as long as there is prejudice, ignorance, inequality, I'll be there."
Natural hazards are this to an extent: as long as there is solar energy, there will be tornadoes and hurricanes; as long as there is vertical slopes, mass movements will occur; as long as oxygen exists, forest fires will continue; as long as radioactive decay continues to warm up the earth's core, earthquakes will keep shaking, etc.
As long as there is life, there will be conflict. This is the founding principle of Buddhism.