Plot mistake?

Why did the killer soap the cellar stairs? It didn't make any sense at all. Who was she trying to kill? The hosts, rutger or his wife? Why? The killer said something like "in the end I couldn't kill rutger too and I chose to go myself to the cellar", but it was the wife who fetched the spirits from the cellar, how did the killer know that the wife would suddenly refuse to go to cellar? Does not add up. Or was it the wife she was trying to kill? Why? She didn't know who the killer was. I don't get it.

Kind of pedestrian effort this movie. Nothing to remember after few days.

Watched because of Tuva, she's amazing.