The mother of this young man who killed all of those people seems like her life should have had some type of intervention before she amassed the firearms and ammunition she was able to get, some of her End Of The World views should have alerted the community.
At the very least, she should have made it impossible for the son to get access to the weapons. She clearly knew that he was mentally disturbed.
She was not DIRECTLY responsible for these killings, but she does bear some of the blame because of her actions.