This could have been really good. An interesting beginning mystery and investigation, a little nice atmosphere inside the monastery and seeing what's going on, with the the mystery deepening a little over time. The protagonist is intelligent and thoughtful, and he's helping to guide an apprentice. There's a bit of discussion about religious beliefs. Nice subtle music. It feels like a well-made film in regards to the cinematopgraphy and does not look or feel old.

And then the story drops like a stone.

And the answer to the mystery is really bad...

[spoiler] The killings were by an old guy... Because of a comedy book?! [/spoiler]

It's very bad and one of the worst ways i can think of to resolve a mystery like this. It's not deep or interesting or fulfilled any scrap of potential.

[spoiler] Such as, maybe the killings were something lovecraftian related. Or a clever serial killer. Or just some other reason normal or not, but at least not an old guy killing because he didn't want people reading a comedy book... [/spoiler]

[spoiler] And to anyone reading this who hasn't seen the film, it's because this is the 1300s, the monk doesn't want people reading a comedy book because he says it's not good for them. That's it. [/spoiler]