Some people are so delusional to the point where they gaslight themselves, it's crazy to witness. At that point you have choose not to play into their delusional fantasy because they will assign you a role in their self serving story. No thanks. They'll hate you for it too lol
The fact they had a bad childhood does not in fact morally absolve them of the suffering they consciously chose to inflict on you. They are no longer a child and do in fact know better, they just do not choose it out of weakness, selfishness or convenience. They have chosen hell.