Man/woman find a way to do what ever mal intent is on his or her mind. Psychological screening works when the obvious is present, who knows for sure what takes place to alter the mental makeup of anyone? Planes, trains, boats, cars, buses, subways, all potentials for disaster. I think in today's world the best we can hope for is not to be around when someone decides they no longer want to live and selfishly take the rest of us with them.
Or perhaps it is not really selfish because who knows if their minds even comprehend what they are doing !!