Why did Catherine pelzer abuse her children?

As a child Catherine Roerva Christen Pelzer (Dave's mother) was abused by her parents, with them subjecting her to similar abuse to which she subjected Dave. She was an alcoholic and had Borderline Personality Disorder (mentally ill): http://thequeenandking.blogspot.com/2009/08/did-dave-pelzers-mother-from-child.html

As a Borderline mother, she selected Dave as the 'all-bad' child and he was the target of her rage. As a Borderline mother, she clearly exhibited the qualities of the Borderline Witch (see Christine Ann Lawson's book: Understanding the Borderline Mother) and, as a result, seriously abused him emotionally and physically.

When Dave was placed into foster home, Russell was then selected as the 'all-bad' child and subjected to the horrible abuse.