Even if there are three pilots, the damage can be done: He may in a violent way take the steering while taking off, or landing. Then, the crew has no time to react. No one can imagine what the man's mind will think to make a disaster. Prevention is the key. Psych tests i mean. Human soul can be turned into abyss.
It's true that airplane is the safest way of transport, but just imagine if the plane would crash in Nice, and not on the Alps. There would be thousands of victims.
It's so sad to see our world becoming so suspicious. We did our best to prevent terrorists and the terror came from the inside of the crew. And we now want securities to inspect the crew.