Some people with depression will consider taking their own lives, some actually do. This man may or may not have been a depressive but he was something else far worse to consider and execute a form of mass destruction.
Obviously intelligent, as he was able to conceal his malady, he would have had little difficulty in amending his plan and tactics in order to cope with whatever security features were in place.
I fear that all the protection systems in the world will fail to prevent such a madman carrying out horrendous crimes. The best protection for the rest of us must surely be an attempt to prevent such people from being in a position where they are able to carry out such action.
At any one moment there are over 5,000 aircraft in the air. Safety of systems has been addressed and to a great degree solved. The training, screening and career potential of pilots seems to have gone in a different direction with a sector of the airlines. How a business can spend £20M on an aircraft and them cut financial corners on the human element beggars belief.