Wiktionary
salt in the wound
n. (context idiomatic English) Something that increases someone's pain.
Wikipedia
Salt in the Wound
Salt in the Wound is a 1969 Italian " macaroni combat" war film directed by Tonino Ricci and starring Klaus Kinski and George Hilton.
Usage examples of "salt in the wound".
But that little ledge, remote and just inaccessible, was fate's last refinement of cruelty, the salt in the wound of extinction, and he knew in his heart of hearts that it wasn't a chance at all, but just a suicidal gesture.
I suppose I'm trained to fight with my hands, not by finding a person's private shame and rubbing salt in the wound.