I don't know, maybe a bit of an overstatement? Modern medicine / psychiatry is pretty good at distinguishing between the odd and the dangerous. The problem is taking the dangerous off the streets before they act. Don't know that society would require the odd to be locked up, too -- we're all odd in our own way(s). Might not be many folks at liberty if we did that! 8~)
How many “odd” people do you have to sift through in order to find the “dangerous” ones? What level of reliability is there in psychiatric diagnosis? How many false negative and false positives are there?
Do we really want to give this much power to psychiatric professionals, many of whom believe conservatives, especially people who like guns, are by definition mentally ill, dangerously so?
What we have here is a case of severe hindsightitis. Like the morons that claimed 911 or Pearl Harbor could have been prevented. Which is of course true, had intelligence analysts lined up half a dozen of the many thousands of snippets of information coming through and drawn the appropriate conclusions.
But in real life, do you have any idea how many “odd” people the cops deal with? About the loud uproar that would follow from “human rights activists” if they started rounding up all who don’t “fit in?”