Sorry. Finger slipped.
The villain's own weapon or malicious plan is the cause of his 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 his own creation, or as small as prankster accidentally setting off his 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.
A "petard" is an explosive device (basically a bucket full of gunpowder) 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.
If an
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 a
Mook, long abused, who finally snaps and turns against him at the last moment, it's
The Dog Bites Back. It's it's a
Video Game boss whose attacks can be redirected back at them, 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.
Compare
Nice Job Fixing It, Villain and
Beat Them at Their Own Game (a video game variation on this). The
Streisand Effect is a subtrope of this, when the petard is censorship.
As this is often a Death Trope, all Spoilers will be unmarked ahead. Beware.