What the story needed [SPOILER]

[Kein]

I'm pretty sure the monsters are from drinking the poisoned 'wine'

Now you make no sense, thsoe ppl DIED there.
Here is what monsters is:

Most of the enemies in the game are in the form of experiments made on humans and animals in the
castle.

[...]\super_secret\design\ancient_game_overview_from_0810.pdf

The book on history also notes monsters that capture people in the surrounding areas, and as we know, Alexander harvests the 'fear' from these prisoners.

Ya, problem is... game TWICE states he fact that it was people who were kidnapping other ppl. First, it was Wilhelm who made that contract wit Alexander, and 2nd - it was Daniel and Alexander himself (scene with family and girl).

Alexander needs the monster servants

That is called as statement. "Alexander needs the monsters". FOR WHAT? There is no logical and reasonable explanation. Because it is a gameplay element without good/detailed setting aspects.