I blame her. She knew her son’s mental condition better than anyone and she made weapons available to him.
The mother might have never imagined that others besides herself would pay the price for her poor judgement.
I wondered after reading about the shooting how that kid knew how to shoot. Later I found out it was his mother who took the kid to the target range and got them to learn how to shoot. “She” trusted her kid with guns. Neighbors who knew the family said that the younger son was likely to committ such a crime: killing lots of people. It seems she is to blame.