I think they're just confusing "understandable" with "justifiable". You can see how a person could've degenerated to the point they abuse their family member(s) but all those circumstances, while they may explain their actions, does not excuse them. It is not justifiable to traumatize an innocent person to cope with your own trauma.
Perhaps this understanding can make you believe she can fix herself if she gets the support to do so but a lot of people don't have much hope for people like this (personally, I don't mind seeing redemption arcs for bad parents in fiction, it's a nice fantasy. However, in real life: I find it impossible to forgive or forget - if I found out you abused a child then you will forever be flammable trash in my eyes)