Please don't say that anyone's child will "probably" be a school shooter, no matter how severe his problems are. No one really can predict that, and you don't help the case of mentally ill youth by making blanket statements like that. Yes, the woman's child is severely ill. Yes, our mental health care system does not have a good way to handle children like him. We need to fix that, but not because they're all going to be school shooters. We need to fix it because it is the humane thing to do. Mass murders are still rare, exceptional crimes. Mentally ill people are far more likely to harm themselves than to commit mass murder.
We don't know yet whether the Sandy Hook gunman was mentally ill, and we don't know what, if anything, was done to treat him. But I can assure you that at 20 years old, his parents could not have, for all the money in the world, gotten him treatment if he refused it. In my state, once a child turns fourteen -- yes 14 -- they control their mental health treatment. No parent can force them into therapy. You can hospitalize them only using the same involuntary commitment rules that apply to adults
The woman in the Anarchist Soccer Mom blog has a son who is still a minor, and even she can't access treatment for him consistently. She is absolutely right that any kids like hers end up in the juvenile justice system when their parents can't control them anymore. They don't all commit mass murder.