…was related to Charles's father.

• Charles is a neurotic guy in general, but it's his father and their complex relationship/perceived-betrayal he's most sensitive about.

• It also makes sense with the next episode being at Charles's sisters, where he/they might learn something new or reinterpret their memories of their dad.

• Charles has been learning new information about him over the seasons, but there are still many unanswered questions about him and his actions.

• The fact that Charles seriously thought that Bunny could be his half sister tells us that it's not out of question that he's got a half-sibling or two out there

  • Edit: As has been pointed out to me, there's a strong case for Charles and Sazz being related. They've got the super rare blood type besides similarities in physique/looks, traits, and taste in women.
  • It was called "Once Upon a Time in the West", which is named after a movie (like every episode this season)
  • In the movie, a rich guy hires a mercenary to kill a man who owns prime estate. But when the man's widow inherits the land, only an outlaw and a mysterious gunslinger can protect her from being killed.
  • Charles's dad owned the Arconia apartment. After his death at a young age (possibly a murder), his other children could inherit it, but Charles currently owns his apartment. So a mercenary may be out to kill him, and Sazz is/was trying to protect him.
  • Even the Pataki Academy had only one valuable asset: the land.
  • Dudenoff could have been any one of these people: Charles's half sibling, unwitting protector, or maybe just a filmmaker interested in his life whose research or camera work gave him access to a secret about Charles that someone is trying to hide (partly the plot of Blow-Up, another movie that's also an episode title).

• Property/real-estate could be a motive for other reasons too. Had it not been for the podcast and the sponsorship Dimas money it brought in at the beginning of Season 1, Oliver would have been forced out of his property, hence the threats to stop the podcast.

• Property/rent issues are already known to be the rationale behind the sus behaviours of the Westies.

• Property and rent/ownership has been a running topic of discussion throughout the show. That's literally the first question Oliver (and then Charles) had for Mabel when they first met. It's constantly a source of anxiety for all as the building members keep raising threats to kick them out, and the worst thing Oliver had to say about the Westies was that they were renters (over the line!). Tim and Jan's apartments were immediately the subject of discussion once they killed and convicted for killing.

  • If Charles and Sazz are indeed related, they stood to own both properties (or share it with the other sister).
  • Sazz's body being cremated ensures that her familial connection with Charles can't be proved through DNA analysis.

• Last but not least, this plays into the theme of doubles, since Charles's father was also called Charles and had many other similarities too.

Edit:

• I was wrong about the apartment belonging to Charles's father.