I love it when players make their own villains.
One time that comes to mind was a few years ago. The party was fighting a vampiric succubus (I was of course being a jackass GM as all GMs should be) and one of the PCs, a halfling rogue named Rhyla, was taken down. The rest of the party felt overwhelmed and retreated, leaving her behind.
So of course Rhyla became a vampire. Who really hated the party for leaving her behind.
When Parisha (the vampiric succubus) ended up ascending to godhood later on, Rhyla became a divinely ranked vampiric halfling rogue with some pretty nasty habits. Like impersonating random mysterious assassins that liked to kill people the party interacted with. Or just about anything else she could get away with that wouldn't make the other gods crack down on her for overly interfering with humanity.