Wait a second, there is a point that is missing here.
I'm not educated on the level of Dr. Meyer but 20 years of being a nurse plus 2+ decades of taking care of a severely mentally ill daughter gives me some insight, maybe more than most clinicians, I've had to live with someone I love slowly slipping away slowly and being powerless while it happens.
In a lot of these murderers cases there was someone who had a mental health background who was aware of how dangerous they were, and if the current mental health system were different intervention, including locked psych units etc. would have stopped it. These are indisputable facts, I can dig up the references if needed but they are all pretty easy to find.
I've been in parent support group meetings where multiple members described being assaulted by their children. Authorities(LE, Mental health workers) were involved at the appropriate times and in the appropriate manner. The parents(I vividly remember one parent who looked like she got into a fight with a biker gang) were powerless to stop the child/young adults behavior.
I've listened as one parent talked about the possible need to shoot his own child when that child was released from jail in order to protect the rest of the family. The child had been quite clear that he intended to kill them upon his release.
You can tell me that there is no test for this and I will agree. You can tell me that to lock people up using our current state of knowledge is a little nuts in and of itself, and I'll tell you you are talking sense.
At some point it becomes obvious, even to a layman, that a certain person is dangerous.
Let me assure you as things sit there is no good way to handle this, and if there were a great deal of these problems would have been handled before there was bloodshed.
Gaily bedight, A gallant knight In sunshine and in shadow, Had journeyed long, Singing a song, In search of El Dorado