Understanding is hardly the "only" potent defense. There are better ways to prevent things like this from happening but they are also pretty unpalatable to modern society.

Holding families accountable for treason worked pretty well in feudal cultures. Same here. Put the onus of responsibility for prevention of tragedy on the family. Failure to report / take adequate measures is negligence.

Also I think you are making the mistake most engineers make about the mentally unwell: that they can be "fixed" if only they were "diagnosed" early enough.

Very little evidence this is the case. You can medicate someone to the point they don't care anymore but you can't really fix bugs in their programming.