Naomi - //How can we differentiate? They were all mentally deranged. //
I disagree.
The levels and reactions to mental illness are as varied and complex as the people who suffer from them.
The 911 bombers would appear ‘deranged’ because of the enormity and savagery of their actions, but that does not mean that they were clinically insane in the medical sense of the term.
Similarly, Mr Lubitz appears to have been in the grip of a psychotic episode, but again I am speculating, which is futile.
You and I can count on some direct and indirect experience, but what fits one situation almost never fits another.
Apart from the use of a plane, and mental instability, all other factors are, and will remain unknown.