This far into the thread and no mention of "Billy?" For shame.
"Billy" was an episode of Angel, and the retarded follow-up to the quite solid "That Vision Thing." In that episode, Wolfram & Hart (the evil supernatural law firm that would eventually become the show's big bad) strong arms Angel into breaking into the depths of hell and freeing one of the people there, someone so bad that he got locked away in a private inferno, secluded from the other denizens. When Angel frees the guy, though, he turns out not to have horns or fangs or tentacles, he appears to be an ordinary guy without any indication of who or what he really is. The episode ends with him free, and zero clues as to what he could possibly have done to merit the VIP treatment in hell.
Good setup, right? No way they'll squander it with a pants-shittingly retarded reveal, right? Think again, my friends- Angel was a generally solid show, but it was also a Joss Whedon show, and few people go full retard like Whedon. As it turns out, the guy (Billy) is part of a Kennedy-esque political family and has super misogyny powers. No, that's not a typo. His superpower is that his touch turns men into Lifetime villains, because apparently all men have some primordial hatred of women that he brings to the surface. The episode goes pretty much how you'd expect, with him finally being shot to death by evil lawyer Lilah Morgan (slay kween!) Man, if bullets did the trick, makes you wonder why they even bothered with the private cell.
"Billy" manages the dubious distinction of being both terrible on its own and retroactively ruining a good episode.