JMO about some of the topics under discussion.
Knife skills are fairly similar to boxing skills. The idea of boxing is to learn to very quickly and powerful land punches on a person's torso. Boxers know which parts of the body to go for, above the belt and below the neck. They learn the same overall movements that someone wanting to train with an offensive knife would learn.
Take away the boxing gloves, put a knife into the dominant hand and you've likely got a fairly skillful knife-wielder. Hold the knife in the saber manner (not the manner used in the movie Psycho) and you've got a person who could rapidly silence and murder people quickly.
The mechanics of what happened are one thing. But the frame of mind of someone who would feel no guilt, shame, or reluctance about the idea of killing people while they slept in their beds is unfathomable and much less likely to occur in a person than the ability to wield a knife.
This killer not only entertained the idea without sufficient negative input from his brain to convince him to seek help, but he acquired the means to do it and did it.
I don't think he cares if he dies or lives, and I think he is likely enjoying this phase of the process, in contrast to the victims (including his own family) in this case. We've seen people like this through history. The ancients talk about them - they are rare. I don't think anyone could have predicted this, nor do I think any family system or general societal system "failed." Nothing could have fixed this murderer, except his own realization that he desperately needed help. And then, he would have had to find that help, which is not easy, as there are no easy "cures" for this type of person.
ALL JMO.