AKAAB
....really like most parts of your reply/input, Sir.
Many F/D crew (incl. alcohol) not really sure what 'normal' is in that day-today environment and put up with a load of 'stuff'.
As any Psychiatrist/Psychologist will tell you...hopefully truthfully....there is no test ever to explain why someone just 'snaps'.
Dysfunction, mentally, can come as either 'inwardly-directed' suicide at almost a moment's notice and 'exogenously-directed' to 'kill them all' within frighteningly short timescales. (Almost common in Post-traumatic stress cases, such as war scenarios, for just one example).
The truth is that despite people saying things like he/she would never do that, unfortunately, we are governed by brain activity and anyone can do anything under certain circumstances that are completely undetectable beforehand.
How the relatives/family/friends react will likely be their eventual understanding of the above truths.