Defeat Equals Explosion

"If I become an evil genius, I'll keep in mind not to replace my blood with nitroglycerin, hydrogen or any other flammable thing, I don't wanna die exploding like the bosses of Ninja Gaiden."

—Payasoplas, on Ninja Gaiden

Sometimes an enemy being defeated just... explodes. Not metaphorically like in the Villainous Breakdown sense, but in a literal, physical explosion.

In the case of robots, this could be justified, as the defeat would probably cause them to short circuit and the spark to ignite their fuel. But this is not limited to robots, since this happens with characters who are not specifically stated to be robots, and in some cases are implied to be organic in some way or another.

Having every enemy explode upon death was more common in the 3rd and 4th generation of console gaming, when it was mainly done to avoid drawing death animations.

Examples:

Anime and Manga

Dragonball Z: This happens a lot. Lampshaded in Dragon Ball Abridged by Team Four Star:

(Vegeta hits the ground, large explosion. Later, he crawls out.)

Vegeta (gasping): Why did I explode?

Evangelion all the way. When the Angel's cores are destroyed, they explode and leave a lake of red gunk.
Fist of the North Star: Except with blood, guts (or just liquid light in the anime). And mostly with mooks.
Sailor Moon: Most Monsters of the Week end up this way, particulary in Series 1. Or crumbling to dust, but mostly after exploding in a flash of light.

Film

Return of the Jedi: The evil Emperor apparently explodes on impact after being thrown down the shaft at the climax of the film.
In the Wes Craven film Cursed, when the werewolf who infected all the other major characters dies, he inexplicably explodes in a burst of greenish energy.
Double Team: In perhaps one of the most awesome endings to an action film: The Bad Guy (played by "Mickey" Rourke) is left standing on an armed mine, with a Tiger, in the middle of a Colosseum. Just as the Tiger is about to claw him, he steps off, where the whole place explodes. Completing the total madness, Jean-Claude Van Damme, Dennis Rodman, and the actor who played Belloq outrun the ensuing fireball, hiding behind a Coke machine, while carrying a baby!
Some of the Godzilla films has the enemy monster explode after the titular monster kills them. The early films don't do this however.

Literature

Demons in Dark Heavens.
Before the prequel trilogy came out, the Star Wars Expanded Universe almost universally had dead Jedi fade away while dead Dark Jedi or Sith exploded violently the way the Emperor did. Later it was retconned into a power they had, a kind of Taking You With Me thing. In the Hand of Thrawn duology, Mara Jade once makes this sardonic remark when she's in need of explosives.

"Too bad we don't have a Dark Jedi handy we could kill. Remember that big blast when C'baoth died?"

Live-Action TV

When you think of this trope, you probably think of Power Rangers first. When the Monster of the Week eats the Finishing Move as the theme song plays, he goes up with an impressive fireball. The whole Tokusatsu genre in general employs this - from at least the 70s onward, Kamen Rider, Super Sentai, Ultraman, Metal Heroes and many more have had monsters that go out with a bang.
- Sometimes the background explodes just because Pure Awesomeness is Volatile. Power Rangers especially. And sometimes, the Rangers will turn and pose after the Finishing Move so the explosion behind them will look like one of these sequences.
  - At least once, doing that didn't pay off: once in Power Rangers Dino Thunder, they blast, then turn and pose... and the monster wasn't dead, and blasted the Rangers in the back.
- Also, despite this trope, every series will have the heroes think a monster dead without seeing the customary kaboom at least once. This counts as Genre Blindness bordering on Too Dumb to Live.
Message From Space: Galactic Battle has all major enemies explode. One even tries to destroy a MacGuffin with his death explosion.

Tabletop Games

Dungeons and Dragons.
- In the Dragonlance setting both Aurak and Bozak Draconians explode when they die.
- Killing a Balor for good in the Abyss results in a quite dangerous explosion.
Villains and Vigilantes adventure Devil's Domain. When the Player Characters kill any of the Devil's demons, the demons explode in a cloud of noxious brown smoke.

Video Games

Metroid: Most bosses in the series will explode. This is somewhat problematic for recurring bosses such as Ridley, who has managed to come back from complete annihilation several times throughout the series.
The Cobras in Metal Gear Solid 3 all die by going boom... except The Boss. In this case, it's explained that the Cobras were equipped with microbombs that would explode when they died, in order to prevent the enemy studying their corpses.
All the enemies in Beyond Good and Evil. Exploding on death must be a quirk of the DomZ's physiology.
Any player in Quake III: Team Arena and Quake Live who dies while carrying the Kamikaze holdable powerup. When dying, (and if they're not gibbed) they will produce a huge explosion which shakes the arena.
Unreal Tournament: The carrier of the Vengeance Relic in matches with the Relic mutator will explode violently upon dying.
In Half Life, any Gargantuas you encounter (the big, hulking aliens that shoot fire at Gordon) will die with a chain of explosions whenever killed.
Mega Man: Just about every enemy explodes upon defeat; partly justified in that they're robots, but an explosion happens regardless of whether or not the type of weapon used would logically cause an explosion or not. (For instance, Storm Tornado in Mega Man X). The only enemies that doesn't explode is Proto Man, he simply stops and teleports out of the arena (though arguably he really isn't an enemy) and Dr. Wily (the machine he's piloting does though) because he falls out and surrenders, heck even Mega Man himself explodes into a billion energy orbs when his bar thingy with about 28 other bars...runs out.
Some boss enemies from Jazz Jackrabbit go boom when killed.
In God of War, the giant lava minotaurs explode when defeated.
This happens to all the bosses in Mario and Luigi Superstar Saga, even if they are alive in the cutscene that follows.
In the Shinobi series, all enemies explode when they die.
Shining Force III had enemies explode when killed.
The Legend of Zelda the Wind Waker: All monsters explode into ominous (yet harmless) purple smoke. Except for the giant Armos Statues, who explode like bombs and does damage when you are too close to the explosion.
The lambent locusts in Gears of War will explode when they are killed. This was justified by their exposure to the emulsion.
The one-hit enemies from Contra series get knocked back shortly before exploding.
The weapon in Moon Crystal makes every enemy explode into 4 waves when hit enough. Even bats and all kinds of monsters.
All the enemies in Actraiser series.
The enemies in Kirby series tend to explode with puffs of smoke and pentagram-like shapes.
In NES Ninja Gaiden games, ordinary enemies explode into 4 fiery waves expanding diagonally.
Enemies in Blaster Master enemies meet the same fate.
Sword in Strider series makes them go boom!
Enemies in Ghosts N Goblins series explode too without any given explanation.
In La Mulana, enemies go down that way.
Enemies in Mole Mania die in a very corner-y explosion.
Gremlins 2, every enemy does a Chain Reaction Destruction upon death.
Every enemy explodes in Mushihimesama series too despite most of them being insect-based.
In Gunstar Heroes, the enemies, whatever they might be, explode.
In some of the Castlevania games, enemies explode upon death or leave a small flame.
In Run Saber, almost every enemy seems to explode upon death.
In Silver Surfer NES game, same thing.
Star Fox 64 is a wonderous explosionfest.
Makes sense for tanks and airplanes in Advance Wars. But foot soldiers? Not so much.
Enemies in Gods explode.
Enemies in Earnest Evans.
Everything you can pick up (including enemies) in Superman 64 explodes. Even boxes.

Webcomics

In The Beast Legion, Gorgorath goes through this in Issue 4.