Promise of the Flesh (2018)

Mild-mannered and modest Sook-young looks like your usual middle-aged woman, but happens to be a man-killing murderess. On a train trip to Mokpo, she remembers an earlier trip as a prisoner on furlough accompanied by a menacing but kind butch female guard. On the train they meet Hoon, a young man who falls for Sook-young. The guard lets them be alone at a bizarre marriage in a hill-top graveyard.