When people do terrible things, it seems reasonable to believe we should have taken steps to identify them beforehand. If we can do that, then surely we can prevent them from doing harm.
The crash of Germanwings Flight 9525 in March, which appears to have been an intentional act, is an example. It shocks us (and understandably so) when a trusted professional harms those who have entrusted their lives to him or her.
So why not identify pilots at risk and take steps to prevent similar events from ever occurring again?
Because it is likely impossible, and maybe even counterproductive.