They know more about the threat than anyone. Can tell the hero/heroine everything they want to know. They're a cop who's been hunting the Serial Killer for their entire career, or the Great White Hunter who knows more about dangerous animals than anyone. Perhaps they're an expert on ancient curses, or are schooled in the ways of killer robots. Whoever they are, they're the good guy's best and most reliable source of information regarding The Threat, its plans, its motivations, its patterns, its strengths, and its weaknesses.
Unfortunately for them, they're not the hero of the story. Even more unfortunately, they're just useful enough to the hero that their death will make our heroes even more vulnerable, and their plight more dramatic. Worse still, since they're the expert on The Threat, their death by the Threat will augment the advantage the villain has over the heroes — after all, if this Expert couldn't win against them, what shot do our non-Expert main heroes have?
The above may be unfortunate, but this is just embarrassing: Not only will our Expert get killed by the Big Bad, but at no time in the confrontation will our Expert even pose a major challenge to the Big Bad. The Expert, (usually late in the game having imparted all their useful advice and wisdom about the threat), will be dispatched neatly, cleanly and almost effortlessly by the villain/monster/demon, making the audience wonder how in the hell this guy even lived long enough to ever become an expert on this particular threat at all.
The two important criteria are that this supporting good guy:
has superior knowledge/training/exposure to the Threat, enough that they can educate the Real Heroes about it, and
when actually confronted with the Threat, they go down fast and put up a perplexingly feeble fight.
Always keep in mind the second one. If our ill-fated supporting hero took out a couple dozen mooks, held their own (but then lost) a knock-down drag out fight, put three barrels in the shark before being eaten, or otherwise put up a decent fight before dying, they do not fit this trope.
Compare/contrast The Obi-Wan, who is also wise to the threat and often killed by it nonetheless, but The Obi Wan's death, if and when it comes, is often a Moment Of Awesome, Obi-Wan Moment, or at least dramatic. Compare also The Worf Effect, which is where an enemy dispatches a more generally established badass with ease, rather than one specialized against them. Finally, consider Mentor Occupational Hazard, which is the tendency of mentors to the hero to die.
As a Death Trope, all Spoilers will be unmarked ahead. Beware.
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Anime and Manga
In the first season finale of Blood+, Saya, Haji and Eric team up with a squad of professional chiropteran (vampire) hunters. They die to a man fighting baby chiropterans.
Puella Magi Madoka Magica: Mami Tomoe is a veteran Magical Girl who introduces Madoka and Sayaka to the risks and rewards involved so they can decide if they want to enlist. She gets eaten by episode three's Monster of the Week.
The protagonist who truly knows the most about the dangers Magical Girls face, Homura, is actually a subversion. She's the only character who not only survives the series, but never dies in any of the shown timelines since she's the reason these, and the other, timelines even exist.
Happens a lot during the Hunter exam in Hunter × Hunter. Placed on the roof of a large tower? The world class mountain climber knows he can scale the tower with ease... only to be eaten by a watchful bird.
FAITH-eilte Heine Westenfluss in Gundam Seed Destiny: a ZAFT special forces pilot who even Athrun looks up to for his skill and test-pilot of the latest specialist mobile suit ZAFT has to offer. Killed in his very first sortie by failing to check his surroundings before rushing in to engage the Freedom.
Robert Muldoon from Jurassic Park "knows more about Raptors than anyone" ... but is pretty quickly and easily dispatched by the raptors once they're loose when they use their standard attack pattern. One wonders if the fact he was a hunter in a Hollywood movie might have something to do with it. In the book, it's Hammond instead, and Muldoon survives the experience after shooting the T-Rex with a rocket launcher, blowing at least one raptor in half with a shotgun and still continuing to be an untouchable badass even after he finally gets so drunk he can barely stand.
Carson Wells from No Country for Old Men is the character most familiar with serial killer Anton Chigurh. Chigurh doesn't even break a sweat in catching up with him. The un-initiated Llewelen is, however much more problematic for Chigurh.
Major Valentina Koslova from The Jackal is assigned to help Preston and Mulqueen track down the title assassin, bringing with her all her useful knowledge about the villain. Still, her confrontation with him doesn't last long.
Charles Remington in The Ghost and the Darkness is the world's authority on man-eating lions, and how to make them dead. One of the lions takes him in his sleep, it seems.
Phillip Fitzroyce, from the Jaws 3D, is described as a skilled hunter and naturalist who has every confidence he can take down the killer shark that is threatening the movie's fictionalized Sea World park. He lasts about five minutes against the beast.
Quint from the first Jaws movie talked a good game but didn't last very long once he actually met Jaws. However, both he and Hooper imply that they had never seen a shark like this before. Also, in all fairness, they'd been waiting and drinking for hours, so it's also clear that Quint is pretty hammered by the time the shark shows up.
The entomologist who knows all about the killer spiders of the movie Arachnophobia goes down like a Red Shirt. Perhaps a little more acceptable than most examples because he's a scientist, not a hunter, but he still wasn't very careful despite knowing how dangerous the spiders were.
Four Jedi Knights set out to arrest Chancellor Palpatine/Darth Sidious in Star Wars III: Revenge of the Sith. They're led by Jedi Master Mace Windu, who had just then been warned by Anakin that Palpatine was a Sith Lord, so there's no excuse for being taken by surprise. And yet Sidious takes them by surprise. Two Jedi don't even parry a single slash from Sidious's lightsaber before immediately dying, and one more gets in a few blocks before being felled. Only Mace Windu survives long enough to offer a decent lightsaber battle. Windu is not this trope. The other three Jedi are.
Predator 2 has an excusable case, as the expert does manage to survive one meeting with the Predator and though he knew about its behavior and senses, did not have reason to doubt his trap wouldn't keep it subdued.
Predators: the expert has defeated several Predators off screen, but still goes out like a punk the second they show up on screen.
Played for (dark) laughs in Feast, where "Hero", the badass-looking character who bursts onto the scene and actually seems to know what's going on, gets eaten by the creatures within seconds of his first appearance.
Then his wife, Heroine, shows up, and dies halfway through the film.
Played for laughs in Club Dread, when the battle-toughened security chief gets offed mid-speech.
Luca Brasi, the Corleones' top hitman in The Godfather, doesn't survive his first onscreen encounter with armed opponents.
Played for Laughs in the Coneheads movie. As Dan Aykroyd is about to be executed by being forced to fight a Rancor-like creature, the prisoner before him says that he's used his years of imprisonment to study footage of the creature, determine a weak spot where he can strike, and trained until he's positive he can slay the beast. When it's his turn, he goes off confidently to victory and immediately gets his head ripped off.
Happens towards the end in Spy Kids 3D where a teen comes in who calls himself "The Guy". He enters the last level before the others do. You'd think a video game expert would last long, but one hit and all 100 hit points he had were GONE.
The ball-collecting scientist in Planet Terror is the only person in the world who knows anything about the cause of the zombie apocalypse - a green gas referred to by professionals in the business as "the shit" - and what might be done to fix everything. Guess what happens just as he's about to tell the viewers and protagonists about it.
This happens in Nighthawks when the heroes' trainer is ambushed by the villain's sidekick.
In The Goonies, the kids discover the bones of treasure hunter Chester Copperpot, who was apparently killed by the very first pirate booby trap he encountered. Rather than be horrified and turn back, the kids manage to use it as inspiration that they've made it farther than the expert.
Hannibal Chau, a man who deals in Kaiju organs, is killed by a baby Kaiju moments after confidently declaring that it was dead. Subverted, however, in that he turns out to be alive in The Stinger.
The Chinese and Russian Jaeger's and their crew also count. For all their Informed Ability as highly skilled and with much more experience than the hero, their on-screen performance in abysmal. They're tasked with defending against two Kajiu's, but the Chinese are dead and the Russian's crippled by just one Kaiju, the second one's only needed to squash their Hope Spot.
At the beginning of World War Z, a young scientist fresh out of college, believes he knows what's responsible for the zombie outbreak. He tells the hero that it's basically mother nature being evil and bragging about it. But like most criminals she left behind a clue to stop her. After the plane touches down at their destination in the middle of a rainstorm, the crew is attacked by zombies. The scientist panics and falls on top of his gun, killing himself. Given that he tripped because of the wet surface caused by the rain, you could make a case for Tempting Fate as well. He was mostly right about a clue, though it doesn't stop the virus; it only gives the people cover from the attacks so they can fight back.
In the first Cube, the group of people trapped in the death maze is quickly joined by Rennes aka "The Wren" a notorious escape artist who has gotten out of six different prison around the world. He introduces the idea of checking trapped rooms by throwing their boots in, advices the others to focus only on getting out lest they turn on each other and promises to get them all out of there if they can keep up. He dies minutes after being introduced due to a molecular-chemical sensor he didn't notice.
In Chernobyl Diaries, Uri, the extreme tour guide and grizzled veteran, is the first person killed after going out of the car alone at night.
Harry Potter: Book 6; Dumbledore, an expert in magic and Voldemort, gets himself cursed by making a foolish move while retrieving Voldemort's ring horcrux. Justified by the fact that, by his own admission, his desperation to see his lost family and beg their forgiveness clouded his judgement.
Mad-Eye Moody in Harry Potter and the Deathly Hallows. He is described as a master duelist and an expert in fighting Dark Wizards, but is killed personally by Voldemort literally five chapters into the book. He doesn't put up much of a fight before going down, either. To be fair, though, he was left in a bad position, had no way of winning the battle, and may not have had a chance to properly defend himself.
Avada Kedavra is unblockable, so he could only have survived if he dodged it. Considering Mundungus Fletcher disapparated, distracting him and probably throwing his broom off-balance, it's not likely he was able to.
Gilderoy Lockhart is a parody of this trope. He's amazing. A genius. He's defeated evil monster after evil monster- and his written works on how he did it has made him famous. Oddly enough, he seems rather hesitant to do anything in real life. Turns out that he's a total fake, not an expert at all. He's been stealing the defeats of evil monsters off other people, and erasing their memories, that being the one form of magic he has any talent at all for.
In The Dresden Files, Senior Council Member Simon Petrovich is the world's foremost expert on vampires. He doesn't even get an appearance in any of the books, as he is killed off screen by the Red Court during their initial attacks against the White Council.
Live Action TV
Bones: The foremost expert on the serial kidnapper/killer "the Gravedigger" is killed by the Gravedigger.
Kendra, the new Slayer, is introduced as a key ally of Buffy with skills comparable to hers. Then she doesn't show up again until the end of the season, when she's dispatched by Drusilla with very little effort.
Jenny Calendar's people had handled Angelus before and she was the only person who had the curse that would restore Angel's soul, thus ending the threat. He knew this; she knew he knew this. (He'd already killed the other member of her clan in town.) Yet when Angelus came for her, she was completely unprepared.
In "Aliens of London", during an alien crash landing in public, the world's greatest experts on aliens were called together — only for it to turn out to be a trap to kill them all. Subverted, since the Doctor was one of them, and survived thanks to his Bizarre Alien Biology.
The Torchwood Institute, which is supposed to be Earth's best defense against aliens, keeps getting completely wiped out by them. Children of Earth has a particularly nasty example: after much fuss from Torchwood and its allies about letting the alien experts do their job, Jack and Ianto charge in and threaten the aliens... with no plan at all. It goes exactly how you'd expect.
LOST: Most of Arzt's speaking lines in his last episode are about how dangerous and volatile the aged dynamite is, so naturally he dies doing exactly what he told the others not to do with the explosives.
Psych: Similar to the Bones example, this show has Mary (short for Marion) get killed. This was after he theorized that Mr. Yang, a female serial killer who he was an expert on, may have had a partner. He is right, and it's Mr. Yin who kills him.
Star Trek: Enterprise: Crewman Daniels turns out to be a temporal agent of 31st-century Starfleet, sent back to counter enemy agents — specifically Silik, a bad guy Daniels suspects is also aboard. He is. He kills Daniels easily. (Daniels does show up later, having apparently only been killed "in a manner of speaking", but whatever happened, it took him out of the action and left Archer to fend for himself.)
In the episode "First Person Shooter", the owner of the VR game brings in "world reknowned hacker" Daryl Musashi that even the Lone Gunman drool over like fanboys. The game AI murders him by cutting off his hands and head before he fires his first shot.
Given the bodycount of most X-Files episodes, it's not surprising that this trope is played straight in a lot of them.
Neelix on Star Trek: Voyager claims to be an expert on many things, including mining and camping. The former ended up with him falling down a cliff and pulling Torres down with him. The latter led directly to the deaths of at least two crewmen.
He also claims expertise in crew morale and cooking. This has led handmade food on the ship to be viewed as a punishment as well as the direct cause for a thriving replicator credits black market. He has also poisoned the ship on one occasion - not the crew, the actual ship itself.
In The Witcher, Raymond Maarloeve, a private eye motivated by the death of his family at the hands of the organization known as Salamandra, had amassed information on the group and its head, Azar Javed, with the hopes of bringing them down. Raymond decides to aid the protagonist, Geralt, in dismantling Salamandra. The endeavor doesn't end well for him, despite Geralt successfully defending him from Salamandra's initial backlash.
Azar Javed himself kills Maarloeve, and begins impersonating him using magic, which Geralt may or may not discover early on, depending on the player's actions.
In Assassin's Creed: Brotherhood, one Borgia Captain claims, in Enemy Chatter you can eavesdrop on, to know how Assassins operate and thus how to deal with them, but other than being on horseback (and thus being able to flee faster), he's no harder to kill for all his bluster than any other Borgia Captains. As a result, players have been known to time their kills of him to maximize the dramatic irony. For example...
Tomasso di Viterbo: The assassin will not appear in front of you. The assassin will come out of the shadows and—
Ezio: [rushes on foot right up to Viterbo's side, pulls him leg first off of the saddle and stabs him through the throat with the Hidden Blade as he lands]
Inverted in Final Fantasy X, as Auron, the party's leading expert on Sin, knows so much about it because he was killed by Sin's emissary Yunalesca several years ago. So he's the leading expert because he was killed.
In Baldur's Gate, Gorion, your mentor and foster father, is killed at the end of the tutorial level.
Neverwinter Nights has an entire academy slaughtered in the tutorial level, including all of the NPCs that gave you your training exercises.
In Metal Gear Solid 4, the Metal Gear RAY, which was specifically built as a counter to the Metal Gear REX models being built by the other countries, is destroyed by none other than the (half-demolished but operational) original Metal Gear REX.
In Exile/Avernum 3, the first expedition to the surface is composed of hand-picked and trained members of the underworld's military and equipped with items personally made by a brilliant (if profoundly weird) archmage, and end up dead in various corners of The Empire within a few months. Your band of adventurers, with standard military equipment and limited support, saves the world.
Far Cry 3 has the protagonist's older brother Grant. He's a military reservist, which comes in handy after he, his brothers and some friends get captured by Ruthless Modern Pirates who plan on selling them into slavery. After escaping from a prison camp along with the protagonist, he's immediately killed by Vaas.
In the first mission of Mass Effect 1, Shepard is supervised by an elite Spectre named Nihlus. He's killed by Saren when he had his back turned halfway through the mission. Justified in that Saren is Nihlus's old friend and former mentor (thus, someone that Nihlus implicitly trusts) who, unbeknownst to Nihlus, has recently made an off-screen Face-Heel Turn.
Parodied on a The Simpsons Halloween special when dolphins were plotting to recapture the land. The sea captain monologues about how he's the only one who knows how to stop them and is Killed Mid-Sentence.
Parodied in American Dad! where the helicopter carrying the CIA's top code-breaker crashes as he's arriving at the scene. Any remaining hope then falls on Scott who's "pretty good at the jumble". Scott promptly gets hit with debris flying from the helicopter.
In the Season 3 finale of Star Wars: The Clone Wars, Ahsoka is kidnapped by trandoshan sporthunters, and is released on an island on a jungle moon, as prey. There she encounters three Jedi younglings, led by a girl named Kalifa. It's stated that Kalifa has been held there for the longest time, and has out-lived other Padawan-learners, despite being only youngling herself. She even told the older and better-trained Ahsoka in her face, that she had the experince in surviving the hunt. Naturally she's killed soon after Ahsoka's arrival. Subverted in that, she was expert in hiding from, and avoiding the trandoshans as much as possible. She only died after Ahsoka convinced them to go on the offensive, and they were ambushed while scouting after the trandoshan's base.