Not excusing but explaining behaviour, people tend to self perpetuate their own destructive behaviours.Idk if you're excusing her but at the end of the day she's an adult. There is no excuse
It is not born out of them wanting to be shitty people, but rather out of traumatic or unpleasant experiences.
People who hurt are generally hurt themselves, it is not an excuse to not try to change but a lot of those people really aren't aware of their own shitty behaviour because it is just something they do now.
It becomes ingrained behaviour and changing is difficult, in her case it took someone breaking every negative expectation she had to finally realise she was a bad person and to change herself for the better.