If you're so concerned with pathologically violent people, why not simply screen for them at various reasonable points? You provide mental health services at school to disturbed kids and training for teachers on how to recognize and deal with them. You screen people who have violent run-ins with other people before they go onto bigger and badder things. You run a public education campaign so people can recognize when their associates and familiy members need help -- maybe a few people will recognize they need help themselves.
The problem with screening for these kids that do these types of things is that they are very intelligent, but emotionally underdeveloped.
They are smart enough to say the things that they need to in front of teachers and parents. The parents would have to be very smart
and be able to monitor their child's activities without the child's knowledge. The only thing is that most of these children that do go crazy
and shoot up their school seemed to be normal to their parents, so the parents didn't see a need to invade their child's privacy.