Flying Counsel, the tendency on this forum of denial of the possibility of pilot suicide is nothing new. When MH370 went down, early posts suggesting suicide as a potential motive were rapidly deleted. I don't blame the mods for this - the mods are pilots too and it is a natural human tendency to wish to protect the status of one's occupation.
The statistics from the WHO are that 350 million people suffer from depression worldwide. 5-12% of men will suffer from clinical depression at least once in their lives. A much smaller number will experience suicidal ideation. Pilots are people too and will not be immune.
Unfortunately
copycat suicide - where one famous suicide influences a cluster of similar suicides - suggests that the world may suffer an increased rate of these events, so on this basis the 'two persons in the cockpit' rule is welcomed (even though it leads to its own problems).