The other problem is that Pysch testing is beatable. A would be terrorist or suicide candidate can always beat a pysch interview. The other issue is that even if you did pysch evaluation every year your whole world can fall apart two days later and you have 363 days until your next interview whilst dealing with whatever it is that hasn't worked out.
Plenty of Military types and spies have either switched teams or gone AWOL and they are under much higher scrutiny than pilots ever will be. End of the day you can't control all pilots and second guess what they might be feeling or doing. How do you reckon Edward Snowden's pysch evaluation/polygraph testing is looking right now?
I think two crew in the flightdeck will probably stop a spur of moment suicide but nothing is going to stop someone who is determine to take out everybody.