
Perhaps taking down the villain has resulted in an Evil Power Vacuum, and now even worse guys are fighting to fill the void that the previous villain left, causing even more havoc than that previous villain could have ever dreamed.

Maybe that princess you saved is a cold-hearted tyrant at the head of an oppressive regime, or even a Sealed Evil in a Can. Maybe the so-called villain was actually a Hero Antagonist, and you just unwittingly handed a huge victory to the real villain. Maybe the villain, despite their villainy, was serving some other greater good - keeping the world/universe/nature/whatever politically or literally balanced - and their demise throws things into chaos. Maybe the villain, having been defeated or damaged, is now transformed into a new, ultimate, unstoppable, invincible, angry form, often against the villain's will. Maybe the now-dead antagonist was actually holding back an even greater evil.

but not so fast! It turns out that by the very act of success, they've unwittingly made things worse. The hero has accomplished their goal they've killed the Big Bad, or defeated them forever, or at least scored a major blow against the antagonist.
