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 a 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 [...] [tvtropes.org]