I won't name the airline, and I won't say what in what year, but suffice to say I once worked for an airline with 3000 pilots of which 500 were on some sort of mental health medication to treat a vast array of illnesses/issues, at the same time. The amount of pressure that a Pilot endures, such as - relationship issues, health, financial, shift work, away from home travel, a company continually trying to screw you, time pressures, responsibility of caring for thousands of pax per day, etc etc.
The Germanwings co-pilot (and a couple of others over the years) are an extreme case and rarely top themselves in such spectacular fashion. The risk per flight hours globally is very very small. However the risk is there, it will always be there. So we put a flight attendant in the flight deck with us while the F.O takes a slash, fine, I'm good with that, seriously. But what has stopped the Captain bringing a sharpened plastic ruler onboard with him, it is so sharp it cuts better than a steak knife, he leans over and plunges it through the flight attendants throat. End of story, and another aircraft eats a rocky ravene. Get the point? There will always be 'the one who slips through the system', as no system is perfect. The mitigation for 9/11 has directly contributed to the Germanwings crash.
Could it happen here? Of course it could. Let's hope not.