There's Foreshadowing that Sister Agatha was the sick passenger in Cabin 9. Think: If Dracula is regaling Agatha with the story of how he sailed to England on the Demeter, then how is Agatha in England as well?
What's more, it seems initially greedy of Dracula to eat his way through his "larder" (and it really is). But it becomes a Rewatch Bonus once it occurs he's not being careless because he has a safety net: Sister Agatha, his pawn.
Dracula tries to goad Adisa into facing him so the latter could die an honorable death. We come to find that Dracula longs for an honorable death. Projecting, anyone?
If you know anything about Romanian Folklore, then one knows that (Mr.) Balaur is a type of dragon, hence "Dracula", hence "Son of the Dragon".
Dracula having a weakness to Zoe Van Helsing's blood due to cancer makes little sense until you remember a lesser known vampiric weakness is Dead Man's Blood.
The origins of vampirism as revealed in this series are absolutely terrifying: they slowly lose their humanity are desperate for their existence to end, but only a few very specific methods can truly put them out, and even those won't work if they apply it to themselves. Because they don't regain their mobility until some time after their death, they usually end up getting buried and are doomed to spend an eternity trapped underground, and that is if they don't get cremated first.
It's worse than that: while being killed by a vampire can cause a person to become undead, it is by no means a requirement: some people just naturally have a condition where they retain their consciousness after death, no matter what is being done to their bodies ''Miracle Day''-style. Dracula notes that a single cemetery outside London has at least nine such poor souls, eternally trapped inside their coffins and struggling to get out. No wonder they're called "The Sufferers".
What's worse, Dracuala calls the Sufferers "an undertaker's greatest secret"; this, plus several other comments he makes, implies that this isn't an isolated phenomenon. Just how many Sufferers have there been throughout human history? Who's to say that you, or one of your loved ones, didn't suffer such a fate in that universe?
