Release date: The questioner saw it around the time when The Machinist (2004) was popular.
Genre: Romance (?) + crime (?).
The serial murderer is a very tall, big guy.
He lives in a studio apartment, he works in a factory and he only eats beans, his apartment is well-stocked with beans.
One day, while fetching his mail, he meets a girl who dresses like a fairytale fairy and they fall in love.
The guy decides to quit killing people.
Sometimes it’s briefly shown that the girl is working on some sort of art-project in her apartment.
At the end, the guy finds a note from the girl, urging him to come inside her flat.
There, he finds that she’s committed suicide in such a way that he’ll get charged for murder.
It’s not Liza the Fox-Fairy (2015).
https://www.reddit.com/r/whatsthemoviecalled/comments/1nn892i