Monster Fangirl

Hurray! The heroes have captured the murderous Complete Monster and locked him away, and the day is saved.

So why is he getting all this fan mail?

The Monster Fangirl (and this is almost Always Female) might have the belief that Love Redeems, or she's In Love with Your Carnage (in the worst case), or believes that he's really innocent.

Their fate is usually to be casually discarded with a Hannibal Lecture, to wind up another victim, or to go to jail for the crimes they commit to aid the Monster, often screaming declarations of love all the way.

Depressingly common in Real Life. Compare Draco in Leather Pants for when the fandom does it.

Anime and Manga

 * Death Note: Yagami Light founds over half his schemes on these beings. Amane Misa had some potential as a less-polished monster in her own right, as a less brilliant and histrionic psychopath instead of a genius narcissistic one, but decided to slave her entire being to his will on first meeting, and never wavered, and without that his plan to defeat L would never have begun to exist. His final big scheme also relies heavily on Disposable Woman Takeda Kiyomi, a college girlfriend turned news anchor whose response to the revelation that he is Kira is not visceral horror but infatuation.
 * The rest of the Kira fans who keep him in names and faces to kill (yay fact-checking!) and cheer and all, and also his eventual right arm Mikami Teru, are a bigenderal hydra of this phenomenon stretching across the world. Even after the Villain with Good Publicity is exposed and declawed and killed, he still has a huge cult. This series is founded upon existential despair.

Live Action TV

 * The "fiance" of Nate Haskell in the CSI episode "Targets of Obsession". He has a whole club of Monster Fangirls.
 * This reaches new highs of psychosis in "Father of the Bride", where fangirl Tina Vincent cackles maniacally at her own capture, and spends her interrogation alternately smiling at taunting the cops with how much better Nate Haskell is than them.
 * Baltar's cult in Battlestar Galactica sheltered Baltar after his trial from the many, many people who wanted him dead.
 * In Bones, Serial Killer Howard Epps got married to one of these while in prison. Then he escaped and killed her.
 * Several episodes of Criminal Minds, with "Riding the Lightning" and "The Angel Maker" being the most prominent examples.
 * In "Riding the Lightning", the Serial Killer has a horde of fangirls who call themselves "The Brides of Jacob" and deliberately dress up as his victims.
 * Gender Flipped in Criminal Minds: Suspect Behavior with Veronica Day's Monster Fanboys.
 * In Flashpoint, "Jamie Dee" takes a hostage to steal a policecar. It turns out the "hostage" is one of his fangirls..
 * The killer in Season 6 of Dexter gets legions of online fans, plus a pair of fans who actually aid him in a kill.
 * Red John from The Mentalist seems able to recruit fanatical followers of both sexes.
 * One of the criminals in Lie to Me (a rapist who blinded his victims) had a Monster Fanboy who was so devoted that he married one of the victims so he could be "close" to what had been done to her.

Literature

 * The Harry Potter franchise has Bellatrix Lestrange, tenaciously devoted fangirl to Voldemort, and according to J.K. Rowling, in honest love with Voldemort despite his complete inability to feel love, compassion or even pity for another being.

Music

 * Rose and Valerie in The Beatles' "Maxwell's Silver Hammer" scream for the eponymous serial killer's release.

Stand Up Comedy

 * Chris Rock "You get more respect comin' outta jail than school..."

Western Animation

 * In the Batman franchise Harley Quinn is one to the Joker. More specifically she was his therapist until she suffered a Critical Psychoanalysis Failure.
 * Inque in Batman Beyond had a male version. He helped her escape and suffered And I Must Scream while trying to become like her.

Truth in Television

 * Marriage proposals to inmates.
 * The Manson Family.
 * Serial killer groupies.