One of my colleagues, in a senior management position, and a highly stressfull job, was suffering from depression last year. He was put on medication, and appeared to recover, although missing his zest for life, and his formerly high energy levels.
The general thought was that he could be counselled and assisted back to full health.
We recently tasked him to lead a team on a performance audit in West Africa. En-route he went home to visit his family in Jo'Burg and while there retired to the wine cellar, drank a half bottle of his finest...and shot himself !
At least he wasn't flying a 767 at the time.