Sonnet 120 is about traded injuries, or, to my mind, revenge for a past wrongdoing. The speaker has obviously inflicted pain on his lover and is unapologetic about it. What he says, in plain terms, is: We all make mistakes. I bore your unkindness without making too much fuss about it, now it is your turn to show the same grace. My cruelty cancels out yours, and yours must cancel out mine. (Revenge is sweet, though.) Still friends?
Sonnet 120
That you were once unkind, befriends me now,
And for that sorrow, which I then did feel
Needs must I under my transgression bow,
Unless my nerves were brass or hammered steel.
For if you were by my unkindness shaken,
As I by yours, y’ have pass’d a hell of time,
And I, a tyrant, have no leisure taken
To weigh how once I suffer’d in your crime.
O, that our night of woe might have remember’d
My deepest sense, how hard true sorrow hits,
And soon to you, as you to me, then tendered
The humble salve which wounded bosoms fits!
But that your trespass now becomes a fee;
Mine ransoms yours, and yours must ransom me.