Conflict becomes cruel when vulnerability is turned into ammunition.

Using what someone told you in confidence to hurt them during a fight does damage that outlasts the fight by years. It teaches them that honesty with you carries risk, and that lesson does not get unlearned by an apology afterward.

Whatever is said in anger, keep the vulnerable material out of it. That is a line worth holding absolutely, because everything else in an argument is recoverable and this one tends not to be.