The writers and everyone involved all claim that there was nothing supernatural about the kaiser, but when you look at the movie with that in mind, it really looks stupid.
I hated it at first, but then it grew on me, because when when I watched it again a few times, I noticed it wasn't what the writers say it was. It was a "supernatural" story.
We get the "girl friend's death" stated at the end. Just one hint. There are more hints. But the reference to the devil is real here.
Kaiser was irritated by the number of people who thought they would be better "devils" than he. The "usual suspects" were five, but he replaced the girl in the line up, much as Wargrave replaced one of the ten little Indians in Agatha Christie's novel.
Kaiser found five people who really thought they could deal with him.
He was the devil.
Those meetings that Keaton and his girl friend have, are meant to be contracts with the spiritual Underworld. When Keaton sees his girlfriend in the room with the bad guys, he doesn't make a move, because he knows it's the devil's room. And they've been had. His final knowing smile when he's picked off is acknowledgment.
When viewed this way, the movie becomes a film, and makes sense.