It's commonly believed that the idiom "rubbing salt into the wound" is derived from a literal action of making things worse.
Wiktionary claims the literal act was done for torture:
Derived from the stingingly painful sensation of table salt deliberately being rubbed into an open, bleeding wound, as for torture, interrogation, etc.
A high reputation user at English stack exchange says
There's also the more mean spirited rub salt into the wounds, where the one who's currently winning (or has won) does take advantage of his enemy's weakness to inflict an even more crushing victory.
However a blog post titled The Irony of Rubbing Salt Into Wounds disputes this. It claims that salt in wounds is painful, but that it makes the wound better from a health perspective:
That is because when you have an open wound on your skin, it already hurts. When you add salt, it would sting so bad and hurt even more. ... Then I remembered the only the reason why I added salt to my wound was that it would actually help the skin heal faster.
In eras before modern hygiene, did putting salt into a wound make the wound worse overall, either by making the wound more severe, or by having no health effects, but increasing the pain of a wound?