You left out Ed Gein in your list of serial killer biographies.I have to recommend the Marquis de Sade's (highly NSFW) magnum opus "120 Days of Sodom (the School of Libertinage)". It's the story of four wealthy serial killers who are looking for a crime that they haven't committed yet. You could easily borrow the extensive character bio from any of the four villain protagonists. Plus, by the end the story just degrades into a laundry list of bizarre and heinous crimes, including 150 unusual ways of murdering people
Other possible sources of inspiration are the full discographies of the bands GWAR, Insane Clown Posse, and Cannibal Corpse, as well as the song "Stay Wide Awake" by Eminem. The novel "Whargoul" by former GWAR frontman Dave Brockie is also a possible source of inspiration.
Also familiarize yourself with the biographies of Albert Fish, Jeffrey Dahmer, Walter Freeman, Josef Mengele, Shiro Ishii, Fred and Rose West, Ian Brady and Myra Hindley, Dennis Rader, Herbert Mullin, Laurie Dann, Ted Bundy, John Wayne Gacy, and HH Holmes. As well as with the general policies of the tyrants Pol Pot, Idi Amin, Tiberius, Caligula, and Francois Duvalier (as well as the Axis powers and the communists (up to and including the modern ones) but that should go without saying)
And, while not as bad as Caligula, I, Claudius really sets the tone quite well, in both the book and BBC TV series. (from the 1970's)
As for GWAR, yes. Also, their wargame, Rumble in Antarctica.
I had an idea for a bardcore filk... of Warren Zevon's Roland the headless thompson gunner...
Roland was a warrior from the Land of the Midnight Sun
With a yew longbow for hire, fighting to be done
The deal was made in Denmark on a dark and stormy day
So he set out for south England to join the bloody fray
Through sixty-six and seven they fought the Cymri war
With their fingers on their bwstrings, knee-deep in gore
For days and nights they battled the Cymry to their doom
They killed to earn their living and to help out the Sussex crew
Roland, the headless archer....
Norway's favored sun,
You can still hear his headless body
moving through the night,
by the tenor twang or Rolands' yew long bow...