But there are hard truths about everyone. The people who know them can weaponise them if the chose to, and wrap it in the disclaimer "I have to be honest".
Which is doubly ironic, because they are being dishonest with the person they are attacking, by pretending their motive is only "honesty", but also with themselves.
(My brother does this. Any vulnerabilities I share with him will be thrown back at me when he is angry. Bless him, sometimes he forgets I told him, and acts as if it is an insight he worked out himself. And yes, I did eventually stop sharing stuff with him.)