Your mother was sexually abusing you if she was putting her finger inside you. Child sexual abuse. That's what it was. And it in unjustifiable.
Her reasons? Why does anyone sexually abuse a child. There are plenty of theories, but I often think we could drive our minds into hell trying to figure that question out. Comparing what your mother was doing to the actions or truly insane (in the literal sense of the words) of a serial killer? I don't know that that is helpful. Fred West was deranged, his words are the hollow meaningless rubbish that one would see only in the very sickest of minds.
Personally, I haven't ever found any healing to come out of focusing my mind on the reasons my abuser acted the way he did. I'm working on accepting that what he did was wrong, never justifiable, never okay. The same goes for your mother. She was abusing you - and whatever mixed up reasons she may have used to justify that in her own mind, she should not have done that.
It is not a parents obligation to break their daughters hymen. Hymens don't need breaking. Parents have no business becoming sexual with their child let alone altering their childs sexual organs.
Parents, or any adult, have a responsibility not to become sexual with a child - that goes for every child on the face of this earth, inckuding you. That is their responsibility, and your mother was violating that responsibility. There is no way to justify it, and no way to rationalise it.
I'm sorry that your mother did that to you. Be gentle with yourself during your healing - your needs, not your mothers, are what matter most.
