You can influence your children, but you cannot 100% control them. Especially once they are adults, but Id argue even before that as well. I think that parents should be held harshly liable if they criminally abetted the shooting in any way, such as failure to follow their state laws and regulations regarding proper storage of a firearm that led to easy access for the shooter etc. But if the parents followed the law and did not negligently or willingly aid the shooter in any way, I dont see why they should be held liable. At a certain point, an individual is responsible for their own actions no matter how shitty their parents are. Some kids are just fucked up. I have no doubts that some school shooters had better parents than a lot of people in this very thread, and their decision to do what they did had more to do with other outside influences (such as being bullied or getting radicalized in an online space) or mental disorders.