Add insult to injury

To make a bad situation worse by saying or doing something additional that is hurtful or offensive.

This phrase comes from one of Aesop's fables about a bald man who swats at a fly that bit his head, only to slap himself. The fly mocks him for adding the pain of his own slap to the injury of the bite. The phrase entered English in the 18th century and became a standard expression.