The characters need to kill something. Whatever this thing is — and it could be a person, an army, a monster, a sentient, malevolent AI, etc. — the characters need to be absolutely certain they've eradicated it and not left it Not Quite Dead. They're trying to figure out the best method of killing it to ensure that it can't survive. What do they decide on? Fire.
Fire is highly destructive, far harder to escape than a traditional weapon, and reduces its victims to ash (or a melted blob, in the case of metal), making it rather difficult to resurrect or rebuild the victim. As such, fire is often turned to as the must-kill weapon of choice.
Not to be confused with Pyromaniac, which is someone who simply enjoys seeing and setting things on fire, or with Playing with Fire, which is when someone has fire as an elemental power. For those who don't have magical access to fire, Fire Breathing Weapon is likely to come into play. Can be used as part of a No Kill Like Overkill scheme. Supertrope to It's the Only Way to Be Sure. Infernal Retaliation is a situation where the fire does not stop its intended victim.
In King of Thorn, this is how Alice deals with the Medusa virus when it takes the form of Laloo. However, the smoke from the fire is what likely caused the virus to spread.
In Mushishi, a village and their resident mushishi finally decide to burn a parasitic plant mushi in hopes of getting rid of it, but after doing so discover that the mushi intentionally takes over like kudzu in order to be burned so it can enter its adult form.
In Nausicaä of the Valley of the Wind, the people of the Valley of the Wind force their conquerers to give them flamethrowers so they can expunge an invasive outcrop of poisonous spores.
This backfires in spectacular fashion in Fables. When Fabletown is attacked by a horde of wooden puppet-men, they have the battlefield strafed with a bit of dragonfire. When Pinocchio sees this, he desperately tries to point out that they're made out of hardwood, and that while they will burn and die eventually, in the meantime they've now got an army of nigh-unkillable flaming puppet men to deal with.
Subverted in Evolution; the military plots to destroy the largest sample of the alien entity with tons upon tons of napalm. However, as the scientists find out right as the plan is about to be carried out, fire makes the thing reproduce even faster; a small sample in a petri dish exposed to the flame of a lit match is enough to make it overtake one wall of the room they were in. Instead, the day is saved with dandruff shampooHead & Shoulders.
The Krytos Plague in the X-Wing Series is so highly infectious that decontamination of a building consists of burning everything inside it with plasma, including burning half an inch of concrete off the walls.
In one episode of Star Trek: Enterprise, an alien race is seen using flamethrowers on people who have been infected by a disease that effectively turns them into another alien species.
Older Than Feudalism: In The Bible, under Mosaic law, fire was the standard, final "treatment" for garments or buildings that had been infected with disease/mold/etc. First, you were supposed to try water. If that worked, great. If it didn't, burn it.
Let the Gaians preach their silly religion, but one way or the other I shall see this compound burned, seared, and sterilized until every hiding place is found and until every last Mind Worm egg, every last slimy one, has been cooked to a smoking husk. That species shall be exterminated, I tell you! Exterminated!
Space Colony has flamethrowers to deal with fast growing plant life and insect infestations.
In Starcraft, this is how the Protoss dealt with Zerg-infected planets. Thanks to the Firebat, it's also how the Terrans deal with Zerglings, Zealots, other Firebats...
Unsorted Examples from the existing page:
In Dan Abnett's Warhammer 40,000 novel Brothers of the Snake, when Khiron killed a fellow Space Marine, he claimed he had been possessed by a daemon and that, since he had not used fire, it had escaped. Fortunately, Priad remembers this when he figures out who it escaped to.
In the episode "Hookman" the brothers have to destroy the things made from the silver from the original hook.
In the episode "Provenance" They have to destroy a Victorian doll made with the locks of the evil child's hair
On one Angel episode, Wes and Fred went after a demon nest with blowtorches, setting the whole thing ablaze.
This is official government policy towards all threats in Warhammer 40,000's Imperium of Man. Aliens? Reduce them to ashes. Mutants? Burn away their impurities. Nonbelievers? "Light your way in the darkness with the pyres of burning heretics." Chaos cults popping up everywhere? Kill the planet. With fire.
The other races are pretty prone to this — almost every race has some sort of flamethrower equivalent, such as Ork "Burnaboyz", who combine flamethrower and armour-cutting blowtorch in one device. They generally tend to be constantly working on deconstruction, lest they get bored and make other Boyz "do the burny dance."
See also the Cult of the Red Redemption, in Necromunda. While the Sisters of Battle manage a healthy 0.2-0.8:1 flamethrower-to-soldier ratio in their various squads, virtually all Redemptionists carry a flamethrower, flamethrower pistol, underslung single-shot flamethrower on their rifle, or all of the above. Even their giant chainswords have a flamer built onto them.
The Imperium's military as a whole gives us flamethrower pistols, full-sized flamethrowers, vehicle-mounted flamethrowers, even Humongous Mecha-sized flamethrowers, plasma guns, and the meltagun, which is an anti-tank microwave.
The Salamanders chapter specializes in fire based weaponry to the point where they get special rules that make them more effective. The Salamanders list is widely agreed to be one of the most lethal army lists in the recent codex.
The only way for most factions inside it to permanently get rid of Orks is to burn the corpses. You can turn them into a corpse using it, but you still have to make sure afterwards to prevent the Orks from rising up again from the spores they drop when they die.
Love Can Bloom summed up why the Imperium loves fire:
Inquisitor Madek: "Man has a love for fire. Napalm, white phosphorus, promethium, oil, gasoline, meltas, plasmas, firestorms, incendiaries, firepower, fireline, flamethrower, fireteam, firefighter, the flames of war, fire, fire, fire. What else could be said to have benefited as much from man as fire? What other element has been defended, nursed, tutored, fed, and loved more than fire? Our cities, our books, our people, our enemies, our friends, our dead, our living, our greatest works and most heinous feats; all of them, fed to the ever hungry flames, and there it is. Years of brilliant minds worked on that one. For warfare, for country, for humanity? No. We wanted to see what fire, raw unchained flame, could do."
Warhammer also has an aspect, as shown by a quote from a respected inquisitor:
The question is not how to separate the innocent from the guilty. The question is how many I can burn.
Confronted by Treemen? Fireball. Being attacked by mummies? Burning Gaze. Taking on anything else? Conflagration of Doom. There's one lore of magic that is based entirely around killing it with fire, plus the various versions of the Lore of Tzeentch (which MUTATES it with fire).
There's a category for Breath weapons, and a special rule for Flaming Attacks. They are often combined. Case in point, dragons, and the Salamanders of Lustria, who are living dinosaur flamethrowers.
And then there's the Dwarf and Skaven entries... the Dwarf Flame Cannon and the Skaven Warpfire Thrower. SkavenbeingSkaven, it's rare for the Warpfire Thrower to be used consistently on the enemy.
This is also useful to deal with Skaven Hell Pit Abominations, since killing them with fire inhibits regeneration and prevents them from rolling on the aptly named "Too Horrible to Die" table. Said table can involve them spitting out rat swarms, or standing back up with damage restored and a temper.
Incendiary ammo in the sequel makes your main weapon have one whole clip of fire bullets, instantly killing common infected with fire while lighting up special infected and watching them burn for a while. You can even use incendiary rounds in a friggin' grenade launcher and watch a horde explode and rain down in firey bits!
Cry Havoc seems to like this trope. Faustus is burned alive, giving him an arm that is possessed... or something. And then the Vatican drops a fuel air bomb on him.
Film: In Outbreak, this is the government response to a local outbreak of a deadly virus. (More specifically, using the fire caused by a fuel-air bomb to starve the place of oxygen. No oxygen, no hosts; no hosts, no virus.)
Film: In The Crawling Eye, the titular alien creatures are destroyed when the Air Force napalms the mountainside where they had gathered. Whether such techniques were necessary aren't really known, though, since it was the first thing the humans had tried.
Film: The final airstrike in Platoon, in response to a request by The Captain to "expend all remaining ordnance on my pos", includes plenty of napalm mixed in with regular bombs. The firestorm almost obliterates everything and everyone left.
Literature: At the end of the Jurassic Park book, there isn't any of that "Let the dinos live in peace on the island" stuff from the movie. The Costa Rican Air Force levels the island with napalm.
In Deus Ex The flamethrower and incendiary phosphor rockets will incapacitate normal humans and the supertough The Men in Black (the latter of which can take a sniper rifle headshot and keep on fighting) with even the slightest graze, turning them into human torches that run around screaming wildly.
In Beast Wars the flamethrower-equipped Inferno once set Tarantulas on fire, who then ran down the hall while ablaze, screaming.
Lit: In John Hodgman's second book, More Information Than You Require, he says of rats: "You must kill them all. Do it with fire." He says the same thing about infestations of Scottie Dogs and... tides.
In Ars Magica, this trope sums up Flambeau's magical philosophy (at least prior to 5th edition).
Dungeons & Dragons: The psionic version: the Pyrokineticist. Always chaotic, rarely good, invariably fire-heavy. They are so fire-happy that a prestige class prerequisite is "must have set fire to a structure of any size simply to watch it burn"
Jaya Ballard, task mage-become-planeswalker. She's appeared on the flavor texts of over a dozen red burn spells, including Incinerate and Inferno, and her own card pays homage to these spells.
"Some people have said there's no subtlety to destruction. You know what? They're dead." "Of course you should fight fire with fire. You should fight everythingwith fire." "Anybody want some . . . toast?" "Yes, I think 'toast' is an appropriate description." "Eenie, meenie, minie, moe . . . oh, why not all of them?"
Chandra Nalaar seems to be the new Jaya Ballard.
Chandra was even taught by Jaya, "When in doubt, use the biggest fire spell you know." Given that herultimateabilities are some of the biggest explosions ever seen in red, she's learned quite well.
And like Ms. Ballard, she has a few good one-liners of her own on the subject:
"Who'd want to ignite things one at a time?" "Spontaneous combustion is a myth. If you burst into flame, someone wanted you to."
Brütal Legend gives us the Baron, a motorcycle-riding pyromanic bad-ass and leader of the Fire Barons. According to Mangus, they were a group of outlaws that burned whatever they wanted and fled when Lionwhyte took over. They return and team up with Ironheade after helping them defeat the Drowning Doom, which gives rise to one of the most boss lines in the game:
Baron: Burn the other guys!
Kyros in Irregular Webcomic! His obsessiveness to "sort out" any problems he faces by casting a huge fireball, killing everything in his path (usually including himself) is a Running Gag.
In Beast Wars there was a character actually named Inferno. Three guess what his weapon of choice was. In addition to his flamethrower, his favorite solution to everything was to make it "BURRRRRN!" setting ablaze not just the scenery but other characters.
"Say the word My Queen and he shall burn!"
A Halloween episode of The Simpsons had Maggie suddenly lose her legs and grow tentacles. They take her to Dr. Hibbert, who prescribes "fire, and lots of it." This is apparently his cure for everything.
Avatar: The Last Airbender: Fire Lord Ozai decides this trope is an appropriate response to a recalcitrant Earth Kingdom. Why bother with normal methods of subjugation when you can just set the continent on fire? Azula suggested it.
Real Life: Quelea are a type of small bird native to sub-Saharan Africa. They are one of the most numerous species of birds in the world. Flocks number in the thousands and are capable of completely stripping grain fields in a matter of hours. How do farmers deal with the problem? They find the trees the birds nest in, wait until nightfall, and use dynamite and gasoline. Boom!
MAN COOK MEAT WITH FIRE. Not "man show fire to meat and then eat it while it still squirts and pulses." KILL IT DED WITH FIRE YUS. "Medium rare" = "good vet could get it up on its feet in an hour or two." That's not cooked with fire. That's THREATENED with fire. I do not season steak. Start seasoning steak and before you know it? You're French.
In "Bart Carney," a father/son carny team takes over the Simpson house. In trying to think of a way to take back their house, Bart, Lisa, and Homer are each keen on fire as an option — one that Marge keeps vetoing.
Bart: I say we set fire to the house, kill them that way. Marge: We don't want to kill them, we just want our home back! Lisa: Well...if we did set fire to the house.. Marge: No fires! Homer: I've got it! Marge: No fires!
In one episode, Homer has to take care of an endangered caterpillar. Lisa researches the species and discovers that it is "sexually attracted to fire". Homer concludes that God must want the species to die.
It is actually a common practice among those who fight forest fires to start a number of monitored brush fires while also cutting down trees. The rationale? Fastest way to get rid of fuel and helps to stop/control the spread of a forest fire by starving it. That's the way nature does it (minus the "monitored" part). In fact, certain types of cone-bearing trees need fire to open the cones. Many of the worst fires were that bad because environmental groups got their science wrong and sued to stop/prohibit thinning. Area grows into a tangle of underbrush and deadfall, lightning strikes, huge zone of flamey badness ensues. The Black Saturday bushfires in Victoria, Australia in January 2009 were partially caused by this. That, and all the arson-lit fires.
The back fire (starting a fire here so that the encroaching fire doesn't have fuel once it gets here) was invented on the ground. A group of fire-fighters were running from a blaze and one of them realized they couldn't escape that way. He stopped and lit a fire, urging his mates to join him. They told him he was crazy and kept running. He was the only one to survive.
Contrary to popular belief, witches weren't burned in England (or the Colonies, which mostly followed English practice). In Continental Europe, witchcraft was tried as heresy, for which the penalty was burning: but the pragmatic English tried witches for whatever they were supposed to have done with their magic, from murder down to theft and destruction of property, and sentenced them accordingly. So while there were hangings for witchcraft, there were also many cases of convicted witches getting a fine or just a stern warning. Also contrary to popular belief, this was also the practice in Europe at large. The Spanish Inquisition, for example, was quick to pronounce 'Witchcraft' as 'Insanity' and refused to even consider charges of it. However, many local courts in Spain brought people up on charges of various counts of witchcraft on their own volition, though hanging was again only reserved for the most serious of cases. Yet another contrariety to public belief, witches weren't persecuted in Western Europe before the Reformation. The Roman Catholic Church denied the existence of witchcraft and was quick to condemn those accusing others of the practise.
During much of English history, there were only two crimes punishable by burning: heresy and treason. The latter was punished in different ways depending on the offender's status and gender: nobles of either gender were beheaded; common men were hung, drawn and quartered; and common women were burned at the stake. Treason came in two flavors: High Treason (treason against the state, including most of what we normally think of as treason, plus sundries such as counterfeiting the King's seal or cuckolding the King's heir) and Petty Treason (murder of someone with lawful authority over you, most commonly murder of a man by his wife).
This was once actually a medical practice. As late as The American Civil War it was, field hospitals regularly cauterized amputated limbs with hot iron. And no anesthetic beyond whisky and possibly opium. Soldiers often feared hospitals more than death, quite naturally. It did significantly reduce the chance of dying from an infection, but many soldiers didn't care.
Interestingly, in Egypt, people really did use mummies for kindling, since when there were thousands upon thousands of mummies, they made for a ready source of fuel. Their historical value wasn't considered at the time. Could be a small amount of Fridge Horror there.