Before we begin, I just want to point out that these lists are not of my making. I found this on the wmg section of Hazebin Hotel on TV Tropes.com.
Damned souls
A capitalistic businessman who indulged in questionable practices to meet goals.
A Nazi/Neo-Nazi.
Bonus points if Hitler makes an appearance or at least gets a mention.
A nasty Alpha Bitch or Barbaric Bully who died while they were still in their teens.
Charlie's apparent rival: Helsa, seems to be this.
An overwhelmingly racist/sexist/homophobic/transphobic/ableist bigot, possibly Katie Killjoy
A cannibalistic serial killer who's an expy of Hannibal Lecter.
An animal abuser, a poacher, someone who delights in animals' suffering, or someone who REALLY likes animals.
A gluttonous Fat Bastard.
A Domestic Abuser.
An entire Big, Screwed-Up Family, bonus points for them being nice normal people individually, but going to hell because they brought out the worst in each other and weren't able to live apart.
An insane monarch or dictator.
A rapist.
A pedophile.
A Knight Templar. Maybe they could even be The Fundamentalist.
A Satanist.
A violent cult of religious extremists.
A Creepy Child or Enfante Terrible - sadly, kids can be evil too, but I fear what people do to them, more than the kids themselves.
A Mad Scientist, possibly Baxter.
A pathological liar.
An arsonist or pyromaniac.
An Anti-Villain.
A(nother) Fallen Angel.
A cruel ruler.
A political extremist of some kind.
A scam artist who opens a competing (fake) redemption service.
And last but not least, a schoolyard bully
Individual people
Jeffrey Dahmer is already confirmed to exist in the Hazbin Hotel universe.
The axe-woman might have been Lizzie Borden.
Adolf Hitler - he has been in a lot of fiction's depictions of Hell (ex. Little Nicky, The Book of Mormon) so it's very possible if not likely he'll get an appearance or at least mentioned.
Ghengis Khan
Vlad the Impaler - His demon incarnation would have a vampire motif
Jack the Ripper
John Wayne Gacy - he'll be depicted as a clown demon
Ted Bundy
Heinrich Himmler
Bonnie and Clyde - since we’ve seen demons who were obviously Western outlaws in life.
Rollo the Walker - for some context. https://www.youtube.com/watch?v=IOnjG7ocZmI and The House of Normandy.
Judas Iscariot
Zeus
Faust
