Great discussion here, with so many points Itoagree with.
Nightowl, I appreciated your ' cat amongst the pigeons' in relation to families with a number of children, where only one child is "difficult".
I've met many kind, loving families, with only one of a number of children who has become dependent on substances, or become involved in crime at various levels. This includes families close to me personally, as well as the many families I spoke to when working, who had one child/sibling living a chaotic/substance misusing/criminal lifestyle and causing such distress to their loved ones.
I agree with when and others, the vast majority of the damaging parents, individuals I have worked with, who hurt others, have themselves been badly hurt in childhood.
I've also known many people who have experienced dreadful abuse in childhood, but are determined never to abuse others. Complicated, isn't it.