A very interesting way to explain. Nice Job:

"In the medieval days knights and guards roamed the country side/roads/bridges/highways/forests/markets/towns/castles to make sure there was order kept. We are still the same humans who evolved over millions of years, we still need guards and people who will protect us."

But what id 4% of those "knights and guards" are mentally unstable:-(

We are never going to be able to identify all the people who are mentally unstable and are also dangerous. Mental breakdowns happen at different stages of life, some happen slowly, others are triggered suddenly. It does not mean we should not try and do a better job - but I agree we should not view it as the silver bullet.