Board to Death

"Albert Dumfries: Excuse me, Prime Minister. Do you mind my asking...what is that? The Master: [muffled] It's a gas mask. Albert Dumfries: I beg your pardon? The Master: [takes off the mask] It's a gas mask. Albert Dumfries: Yes, but, um, why are you wearing it? The Master: [muffled] Well, because of the gas. Albert Dumfries: I'm sorry? The Master: [takes off the mask again] Because of the gas. Albert Dumfries: What gas? The Master: [muffled] This gas."

- Doctor Who, "The Sound of Drums"

The villain has formulated a plan so dastardly and Machiavellian that the audience is unlikely to pick up on it on their own. Naturally, the villain wants to keep the details from the heroes, but it is important that the audience know what's going on. In this case, the villain will discuss his plan with a random group of subordinates, a group of business partners (or partners-in-crime), hostages, his family, or some other group. He will gleefully lay out every last detail of his plot with an evil glee.

And then he kills each and every member of his audience off in some pre-arranged fashion (usually a deathtrap of some sort). That way, the audience knows of the villain's plan, but there's no one who can betray the villain by talking to the hero.

Related to You Have Failed Me, You Have Outlived Your Usefulness, Thrown From the Zeppelin and Bad Boss. In part, it plays to the fantasies of anyone who's been in an annoying or boring meeting, and the desire to kill their clueless bosses.

Anime & Manga

 * During the Dark Tournament arc of Yu Yu Hakusho, Sakyo reveals his plan to open a gateway to the Spirit World to the rest of the Black Book Club and then has Toguro kill them when they object.
 * Bleach had a version of this, although it's debatable whether the board (The Central 46) ever found out what the Big Bad Aizen was planning before he killed them all and started giving out orders in their name that favored his plans.
 * Happens in at least one arc of Umineko no Naku Koro ni with the first twilight.
 * Set up, then subverted at the start of Vexille. The Dragon gives a very broad outline of a plan, then the board dies from poisoned drinks.

Comic Books
"Council Member: (choking on chlorine gas) Starr -- No -- Don't -- You kill the Grail, you damn the world -- this is about mankind's salvation... Starr: This is about my genitals. Go on to extinction, holy men."
 * Preacher (Comic Book): Herr Starr gives up on the Grail's plan for global domination and devotes all his energy to killing Jesse Custer. He arranges a video conference with the high council and tells them all about his plan to undermine the Grail from within. Then he lists off all the indignities he's suffered in pursuit of his goals, culminating in him dropping his pants and revealing the tube where his penis used to be. Having (literally) revealed everything, Starr gasses the council and readies his troops for a final showdown with Jesse by the Alamo.

"Luthor: Splendid. And this? Minion: WAIT! Uh-- heh-- that switch apparently causes the chairs around the conference table to flip backwards, dumping its occupants through a floor panel down through a chute into a vat of acid, sir.
 * Ozymandias does this with his lab assistants in Watchmen. In a subversion of the trope, his speech reveals nothing about the plan itself (which is saved for The Reveal to the heroes themselves) and is merely a partial retracing of how they've come to this stage and a praise to his subordinates for the integral part they played in it -- after which they are killed off painlessly because they knew too much and not because Ozymandias felt the need to tell them what they already knew. Furthermore Ozymandias clearly considers the deed Dirty Business and explains (to their drugged and soon to be frozen to death bodies) that he feels ashamed at having to reward his faithful retainers with "such an inadequate reward".
 * Parodied in an issue of Action Comics, in which Lex Luthor, whose building (along with the rest of Metropolis) was recently replaced by a 64th century version of itself by Brainiac 13, is being shown what all the buttons on the control pad next to his seat in the LexCorp board room do:

Luthor: This is a joke?

Minion: No, sir. We lost two men today before we restudied the schematics.

Luthor: Hmm. Just when I thought the Luthors couldn't stoop any lower, my successor installs Death Traps into the board room. I love the future."

Film

 * Goldfinger, in a scene apparently staged for the sole purpose of allowing James Bond to sneak in and overhear it through a hole inside his model of Fort Knox.
 * Subverted in that the plan he told them wasn't the real one.
 * In A View to a Kill, when Zorin spelled out his plan to monopolize the computer industry by destroying Silicon Valley, one subordinate wanted out. Zorin requested that he step outside for the remainder of the meeting. Unfortunately the meeting room is in a zeppelin and the only exit leads to a long fall. Later, Zorin betrays the rest of his "partners" and everyone else who worked for him in this scheme(the story was largely borrowed from Goldfinger, simply replace "Fort Knox" with "Silicon Valley").
 * The first Spider-Man movie. In this case, the board members are killed because they were trying to oust Osborn from the company.
 * The assassination of the Romulan Senate after they refuse Shinzon's proposal in the beginning of Star Trek Nemesis probably counts.
 * In the 1989 Batman movie, The Joker kills one of the crime bosses with a trick joy buzzer and has his gang herd the others out with tommy guns. He then later kills another boss with a quill pen in the throat (presumably poisoned). As for the rest of the board, it's implied that Joker ultimately decides to "grease 'em now" offscreen...that's just how he rolls.
 * The original script called for one goon to take out another with a flamethrower disguised as a helium container.
 * In Dogma, Loki lists off the various sins of Mooby Inc.'s board of directors before executing them in general for idolatry -- with the exception of one woman, whom he declares to be an innocent and leaves unharmed. Bartleby does have to stop him from killing her, too, because she didn't say "God bless you" when he sneezed.
 * Loki might have been joking about that.
 * Granting the supposition that the sneeze wasn't the least convincing acting Damon's done in his entire career, Loki probably faked the sneeze so he could do that, and the otherwise nice lady just assumed (rightly) that it was fake and didn't warrant a blessing.
 * The film version of Dick Tracy shows gang leader Big Boy Caprice holding a meeting with several of his former rivals, trying to convince them to join forces under his leadership so that they can more effectively extort protection money from all the local businesses. But one of the rivals, Spud Spaldoni, wants to remain independent and opts out. Caprice's men immediately pull their guns on Spaldoni, but Big Boy waves them off and apparently allows Spud to leave. Spud and his entourage exit the building and climb into their car - and then, as soon as the key is turned in the ignition, the vehicle explodes in a massive fireball. Everyone still at the meeting rushes to the window to watch in shock, while Big Boy deadpans: "Very upsetting." (The implication, I guess, is that Caprice had somehow suspected that at least a few would-be business partners would try to back out, and had had his men rig every single gangster's car to explode.)
 * Played for Laughs in the Russian comedy Balls of Fate. The Big Bad of the movie is an oligarch in possession of the eponymous artefact that allows him to warp reality in his favor. Of course, he can't resist showing off such an asset to his guests, but at the same time can't allow anybody to know. The solution? He invites some people, brags about his Balls to them, and then has his Dragon kill them all.

Literature

 * In the Star Wars Expanded Universe novel Darksaber, Admiral Daala gets all the Imperial Remnant faction leaders together in a meeting and then gases them all so she can assume control over all their forces. She does make an honest effort to tell them her plan and get them to sign on willingly; since Imperial leaders strongly tend to be short-sighted egomaniacs, it rapidly becomes clear that it'd be easier just to kill them all and do it herself. When she did and then failed miserably in her campaign against the New Republic, she fled in a huff and left the Empire to Captain Pellaeon, the shining exception to the rule, who only killed board members if they attacked him.
 * In the short story "Nemesis" by Arthur C. Clarke, the Master has been defeated in his quest of world domination, and his army is being destroyed. He gathers his cabinet at their underground boardroom for the last meeting, even though they all know that the war is lost, and surrender is unthinkable (and wouldn't work anyway), so they'll have to die here. However, he reveals that he plans to put himself into suspended animation for a hundred years, then begin his plans again. He then leaves, welds the metal door closed, and blocks all tunnels behind him with cave-ins.

Live Action TV

 * Happens in the Doctor Who episode "The Sound Of Drums", where the Master kills the entire Cabinet in his role as Prime Minister. In something of a subversion that plays even more to office fantasies, he doesn't explain anything; he just insults them (hilariously) and then kills them.
 * At least he tells them why he kills them before it happens. And not in a nice way.
 * Ba'al did this to all (Or is it?) his clones in the second-to-last episode of Stargate SG-1. They had all been cooperating surprisingly well until that point; it seems he only did it because the writers had to figure out a way for him to be killable for real.
 * In RoboCop: Prime Directives, Damien spearheads the project for developing a fully-functional AI. However, the OCP board decides to shut down the project and fire Damien. Furious, Damien goes to the AI control console and orders the prototype AI to gas the board room. He later gets his comeuppance when the AI turns on him and cuts him in half with Frickin' Laser Beams.

Video Games

 * Command & Conquer: Generals 2 as a backstory, has almost all the world's politicians and diplomats killed by the GLA during a Peace Conference, which leaves the Generals and other military figures in charge.

Truth In Television

 * Unsurprisingly, Hitler. A little over a year after coming into power he had most of the leadership of the SA (the Brownshirts), as well as allies of Franz von Papen (thought not von Papen himself) killed in the Night of the Long Knives. The SA included most of the oldest members of the Nazi party, and von Papen was the one who had convinced the prior German government to form a coalition with Hitler in charge.