The villain's own weapon or malicious plan is the cause of their downfall and/or death. This could be something as big as a Mad Scientist who creates monsters and/or a Weapon of Mass Destruction getting killed by their own creation, or as small as a prankster accidentally setting off their own trap.
In media intended for young children, it is often a Death by Irony that releases the hero from the unpleasantness of actually killing their enemy; a more specific type of Self-Disposing Villain.
If the hero is not responsible for the death in any way, it can also be a Karmic Death. Would also overlap with Accidental Suicide.
A "petard"note was an explosive device (basically a bucket full of gunpowder, and a medieval ancestor of the land mine) intended to demolish gates and fortified walls; being too close to the detonating explosive could well toss the engineers who planted it into the air. Thus, this term literally means "Blown into the air by one's own bomb" and was first coined by William Shakespeare in Hamlet, though the original quote was "hoist with his own petard".
If an evil Eldritch Abomination, God of Evil or otherwise supernatural evil entity gets Punched Out by the same powers it gave, you have a Faustian Rebellion in action. If it's an Ancient Conspiracy, Government Conspiracy or other organisation whose Applied Phlebotinum is empowering the one who will bring them down, you have a Phlebotinum Rebel. If it's done by making him hit himself, it's Stop Hitting Yourself. If it's attacking them with their own weapon, it's Use Their Own Weapon Against Them. If it's a Mook, long abused, who finally snaps and turns against him at the last moment, it's The Dog Bites Back. If it's a Video Game boss whose attacks can be redirected back at them (tennis-style or otherwise), it's Tactical Suicide Boss. Gone Horribly Right is a sub-trope where a Super Prototype works too well, destroying the person or persons who created it and/or threatening the world/galaxy/universe. If it's a vehicle that you just stole, it's Vehicular Turnabout.
Can be the result of Didn't Think This Through, but not necessarily. Compare Nice Job Fixing It, Villain, A Taste of Their Own Medicine, and Beat Them at Their Own Game. The Streisand Effect is a subtrope of this, when the petard is censorship.
For examples involving literal (explosive) petards, look up Explosive Stupidity, Grenade Hot Potato, and Pineapple Surprise.
As this is often a Death Trope, all Spoilers will be unmarked ahead. Beware.
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- This 1993 Energizer commercial features the Witched Witch of the West trying to fricassee the Energizer Bunny, and winding up setting off a fire alarm sprinkler and getting all wet as a result.
- Claymation Dominos Pizza mascot The Noid was as bad as Wile E. Coyote at times, in commercials like this one, this one, and especially this one.
- There was an Erin Esurance commercial where burglars tried to break into Erin's house. Her cat dumps some stuff onto the burglars, causing the dynamite they had set up to blow up on them.
- In the Armen Film Animated Shorts Who Will Tell A Fable? the king's attempts to con his people out of their land by promising half his kingdom to someone who could tell a story he wouldn't believe, and taking all their possessions if they fail, ends up getting him tricked by the young boy into either giving him half the kingdom for a claimed debt, or not believing him and giving him half the kingdom.
- In Firing Range, the inventor is killed when his empathic tank senses fear in him, triggering its attack.
- In episode 3 of Nana Moon, Les sets up a monster plant to capture Keke and Amy. The two never pass by the plant, and when Les goes to check, she ends up wrapped up in vines instead. Grunt and Grumble kick the resulting vine ball around and into the sky, unaware that Les is trapped inside until they see Keke and Amy are still out and about.
- In Pleasant Goat and Big Big Wolf, Martial Wolf tells his pack to starve themselves so that they can become thin enough to enter Goat Village. They all starve to death.
- A common Vietnamese proverb: "Gậy ông đập lưng ông" (The man's stick hits the man).
- Invoked in Tales of the Arabian Nights, when the player hurls the evil genie's fireball (pinball) back at him.
- Occurs in Zen Studios' Spider-Man pinball, where defeating the Green Goblin requires hitting him with his own Pumpkin Pinball.
- At Universal Studios:
- King Ghidorah got hit with this twice over the course of Godzilla vs. Evangelion: The Real 4-D. First, using the very building he was levitating, the Children pilot their EVAs to use this against him, with Shinji ultimately stabbing Ghisorah in a wound Godzilla opened. Then, Ghidorah tried to kill Godzilla with his Gravity Beams, only to reenergize Godzilla and trigger a Beam-O-War that results in Ghidorah's death.
- Miseria in The Eighth Voyage of Sindbad when the titular hero uses her own Sultan's Heart weapon on her.
- In BIONICLE, the Bohrok-Kal are about to release their leaders, the Bahrag, with the Toa's own Nuva Symbols when the Toa use their abilities to cause the Nuva Stones to start feeding power to the Bohrok-Kal. At first the Bohrok-Kal are overjoyed at this, thinking they were going to be powerful enough to rule alongside the Bahrag but soon they realise the Nuva Symbols were giving them too much power and, inevitably, they lose control of their abilities and are destroyed by their own powers.