"Cut it out! Stop wasting your damn bullets, you jerks! You need to hit their heads! I told you! See, like this!" (shoots zombie in the head)''
—Hell of the Living Dead
The ubiquitous "gotta shoot 'em in the head" scenario. For many different reasons, The Undead throughout fiction are vulnerable only to Boom, Headshot. Kinda like how (many) vampires are only vulnerable to getting stabbed in the heart. Fortunately, removal of the head also works on people who are not a member of the walking dead, so you don't have to worry about it going out of fashion as a killing method. This may have a somewhat scientific basis, as a zombie would have a lowered to nonexistent pain response, and would most likely not stop attacking until its nervous system has been effectively severed. Of course, doesn't explain why excessive blood loss and blows to the spine aren't as effective...
Is a consequence of a Cranial Processing Unit.
Sister trope to Boom, Headshot and Attack Its Weak Point. It is distinct from them, however; Boom, Headshot is when a headshot is the most efficient way to kill something, whereas this trope is where Boom, Headshot is the only way to kill something; Period. Also closely related to and frequently overlaps with Decapitation Required, but the latter trope usually applies to zombies, vampires etc with a supernatural origin as opposed The Virus.
Most songs about zombies tend to lampshade this trope.
Claymore. Due to Yoma's amazing Healing Factor, a quick kill is absolutely necessary, and even then, it's preferred to completely tear their corpses to bits. Ophelia lampshades this, telling the Awakened who breaks her neck that you need to behead Claymores to be sure.
In Mermaid Saga, some people who eat mermaid flesh turn into zombie-like monsters and some become immortal. In either case though, the only way to permanently kill them is to decapitate them.
This is how you kill vampires in JoJo's Bizarre Adventure - the mystical artifact that creates them does so by altering their brains, making destruction of the head the only way not involving sunlight or Hamon to get rid of them. (Decapitation just creates a pissed-off vampiric head, so you have to destroy the head entirely.)
Played mostly straight in Hellsing. The ghouls will only stop going if they're shot in the head, with which the power of the guns thecharacters are using, usually destroys the head completely. It's been stated that they will stop if shot in the heart, but we only see it with the vampires controlling the ghouls. Killing them this way is seen as preferable; the victims who become like this had no choice in the matter and are brainless, flesh-eating machines.
Subvered in Fullmetal Alchemist with the zombie-like "mannequin" homunculi. While destroying their heads does keep them from biting, they stay alive and it's much easier to disable them by injuring their legs.
Comic Books
Marvel Zombies can only be killed by destroying their brains. Decapitation just leaves an irritated head and a lifeless body (as shown by Zombie Wasp and Headpool).
Except for Earth-Z's Electro, which was his headless body walking around.
Fan Works
In the Naruto fanfic Time And Again Naruto fights a rogue ninja with incredible regenerative powers. He's able to take him out of action by cutting off his head, but is Genre Savvy enough to watch it for a few minutes afterwards and sees the flesh of the neck wriggling as it begins to try and grow a new body, so he pulverises the brain, which finally kills his opponent once and for all.
Resident Evil. The zombies can only be killed by severing the top of the spinal column or massive trauma to the brain.
Literature
Zombies from The Zombie Survival Guide and World War Z are vulnerable only to headshots because the virus causes radical mutation, making everything but the brain completely vestigial.
Partly averted. Removing the head renders the zombie harmless, but not destroyed. The head is still 'alive' and can still bite. In fact, the Record Attacks comics recount a rite of passage involving spending the night locked in a room full of moaning zombie heads (presumably, they don't need lungs or vocal chords to moan, although it has been suggested the moaning is psychosomatic
it's expected, so it's fervently imagined to happen
).
A soldier also recounts in World War Z how it is entirely possible to shoot the skull but miss the brain and, thus, need more than one headshot to finish the zombie. The author includes this as a warning for people not to panic that the zombie is invincible if one shot doesn't do it... unlike the men in the soldier's squad that day...
The Vampire Counts book, however, suggests that when you "wound" a zombie, you're actually smashing it into enough pieces that it can't do anything - which, given the state of decay involved in one of these things (they're literally held together with fence posts and nails), is easier than you might think.
Decapitation is actually the only way to kill a vampire in Bram Stoker's original Dracula. (Buffy-style staking is kind of a Plot Tumor of the Dracula-derived vampire mythos; driving a stake through the heart is used to immobilize the vamps in Dracula so it's easier to take the head off.)
Lampshaded in Brains: A Zombie Memoir.
Averted in Counselors And Kings- it's explicitly stated that removing the head does not destroy a zombie, though it does blind and deafen it (since the now-headless undead has no eyes or ears). Magic or completely destroying the corpse (through dismemberment or fire) is what kills it).
Averted in A Song of Ice and Fire: the only way to destroy a wight is to chop them into little pieces (just dismembering them is not enough, the severed limbs will come after you) or burn them.
Geeks in The Walking Dead; downplayed when the group cuts the head off a walker, but the head is still alive. Daryl comes along and shoots the head and comments that only a headshot through the brain will put them down. This is also true in the Graphic Novel series it's based on.
In Supernatural, you kill a vampire by beheading it. One episode has Dean kill one by shooting her in the head.
In Resident Evil, only the mid-brain has been reanimated by The Virus, the bit that controls motor functions and hunger.
However, they don't need to be shot in the head to kill them; it just gets the job done faster (such that weapons that allow you to specifically explode heads are extremely valuable). Too bad the classic gameplay style doesn't allow basic handguns to be aimed at the head.
It should be noted that in the Gamecube "REmake" of the first game, if the head isn't removed (or the corpse burnt using limited supplies of fuel or incendiary ammo), "dead" Zombies mutate into Crimson Heads. Crimson Heads, in turn, mutate into Lickers, although this has never been a gameplay mechanic.
Chief Irons calls this trope out nearly word-for-word in regards to the mayor's daughter, an apparent zombie victim killed by Irons himself; he just blamed the zombies.
This actually gets subverted in Resident Evil 4 and 5. The "zombies" of the game, the Ganados/Majini can often be stunned by shooting them in the head, but doing so brings risks. Namely, destroying their heads may release difficult to kill Las Plaga, some of which can kill the hero in one hit. The best option is to shoot them in the body, legs or head once to stun them, then use melee attacks.
Stalfos in The Legend of Zelda games. However, in some games, the only way to kill Stalfos is by using bombs (in order to shatter their bones).
The zombies in Cold Fear. The Exocell parasite nests in the cranium, feeding on the brain.
Averted in Dead Space: Often, shooting a Necromorph in the face just makes him mad.
Subverted, more like. One of the gameplay features is called "strategic dismemberment", where removing or destroying certain parts has different consequences, depending on the Necromorph. The best example of this would be the Pregnant, which can only be killed by taking out its arms. Shooting it in the stomach will almost invariably result in instant death.
Minor zombies in the House of the Dead series can be taken out like this. Bosses have their own weak points (although some are in the head).
Touted by some humans in Fallout 3 on how to kill ghouls ("zombie" being used as a slur for ghouls), even though they die from normal damage just like any other creature would. One particular ghoul wants you to kill certain anti-ghoul humans, and will pay you more for killing them with a head shot. However, only one of them is anti-ghoul, the others have a key for a bunker that has the T-51b, and you can keep the keys and take the T-51b for yourself.
Invoked in Eternal Darkness with Ulyoth Zombies. One has to decapitate them or they will go Action Bomb, however, zombies of all four Ancients will still fight without their heads (and will be momentarily stunned, comically patting their neck stump as if to say "Oi, who turned off the lights?").
Used in the final boss battle in the Marine storyline for the 2010 Aliens vs. Predator videogame.
Triple Subverted in Plants vs. Zombies. Zombies, when decapitated, need one more shot to kill them... or just a few seconds, and then the body falls down by itself.
Not a zombie example, but in Star Trek: Bridge Commander, destroying another ship's bridge (which can be considered a brain), will cause the entire ship to explode, even if the rest of the ship was fully intact. This is subverted with Klingon ships, as it's very easy to blow off the front of it and would be game-breakingly easy to defeat them. Though you have to wonder who's flying the thing...
In Time Splitters 2 the quickest way to kill a zombie is to shoot off its head
Zig Zagged in Halo with regard to the Flood. The after becoming flood, the head is useless and they can lose it with no real problem. But the vital zone is their chest, where the infection form is. After having their chest destroyed, the combat form is effectively killed... until another infection form comes along and reanimates it. The only way to ensure no reanimation is to destroy the corpse (most efficient way is to burn it, but continually shooting/meleeing it also works).
Invoked in Urban Dead; zombies killed by headshot take a 5 AP penalty to their "Stand Up" ability.
Invoked in the Borderlands DLC "The Zombie Island of Doctor Ned". True to their humanoid shape, the zombies' weak point is the noggin. Granted, this is also true of every human enemy.
Averted in Doom 3. Headshots do cause a good amount of extra hurt on everything, including zombies. However, body shots work just fine, and some zombies don't even really have heads to begin with.
This is notably not required in Left 4 Dead or it's sequel, unless you active the Nintendo Hard "Realism Mode" where only headshots kill. This makes sense because the Infected are Technically Living Zombies.
In the Fallout New Vegas DLC Dead Money, the Ghost People generally can only be killed by destroying the head or a limb - if they reach zero hit points but have all parts still intact, they just go unconscious and return to full health after a while, unless you finish them off properly when they're down. If you have the super-mutant Dog with you, he can eat them instead, which is also a permanent death.
In Sniper Elite: Nazi Zombie Army, if a zombie isn't killed by a headshot, it has a chance of rising again after a few seconds. Headshots are also instant kills on the most common enemies, so it's in your best interest to aim for the head as much as possible so as to conserve ammo. There are also minibosses and bosses who can only be hurt by headshots; naturally, they take a whole lot of headshots to kill.
Both lampshaded and averted in The Adventures Of Wiglaf And Mordred. One character (Gawain) is a Revenant, an intelligent zombie. The first thing that happens to him is a headshot. Arthur, who witnessed the event then calls foul claiming Gawain can not possibly be a zombie — only to be corrected:
Arthur: You're not a zombie. Everyone knows you take them out with a shot to the head. And you're still standing. Gawain: Have you ever killed a zombie? Arthur: No. Gawain: Met one? Arthur: No. Gawain: Then, how exactly do you know that actually works?
The zombie wranglers of Zombie Ranch declare the front part of the brain "don't matter", but that destroying the rest is a sure way to put a zombie down or prevent an infected person from turning. If you stay away from the brain you can kill an infected person and they'll still rise again later, which leads to this fateful decision.
Subverted in The Order of the Stick, when Roy decapitates an undead dragon. Xykon points out that he controls it with his mind and it doesn't need eyes, ears or a brain to follow his commands. In fact, the main thing Xykon is annoyed by is that the dragon no longer has teeth to bite with.
Web Original
Invoked in Tasakeru. During a fight with a Made of IronRevenant, Commander Nadeshiko wonders how invincible he'll be with a split skull. She doesn't get a chance to find out.
Discussed almost like a mantra in After Hours when the gang discuss zombies. Subverted by Katy, who's apparently unfamiliar with the rule and mumbles something ending with "... extract Brian."
The Flash: How do we fight it, or them? Mophir: Two ways. Pure light from Mophir's gem drives evil spirits back into Dark Heart. The Flash: Great. What's the second way? Mophir: Separate host head from body.
Frylock: I'm sorry you had to see that, Meatwad. But in order to kill a zombie, you have to separate the brain from the spine. Headless Shake: Hey I just heard, like, a pop. Did you guys hear that? Frylock: (throws away axe) Goddamn Wikipedia!
Real Life
With one exceptionnote Mike the Headless Chicken, either of these methods will kill any animal you could possibly encounter in real life note well, okay, aside from the ones that don't have heads.. (Effect may be delayed in some insects and crustaceans.)
Interestingly, in some insects, such as the cockroach, the only reason decapitation eventually kills them is that they eventually starve to death. No head means no mouth.
This is because a large number of insects have a secondary nerve cluster in the thorax just behind the head, which essentially acts as their backup brain. In an extreme example, this is useful. The common Praying Mantis male often gets its head eaten first by the female during mating but is able to continue copulation - removing the head simply removes its limitations and allows the male mantis's body to continue mating until it dies or is completely devoured by the female.
