As RANTANPLANE said earlier - probably we will never know why....
In the mid-90's when I was out of work, I did some HAZMAT procedural reviews + updates in Russia for a FTSE 100 UK company's operations there....Met the Group CFO ( Finance Director in those days ) a couple of times who was blue-chip background, a top, top businessman, and genuinely nice guy....
Fast forward 10 years or so, and having a fairly new wife and young daughter and having had a fantastically successful business career in finance / investment banking, which had accumulated a personal wealth of £10 million plus, he threw himself under a train near Heathrow....
To this day, nobody knows why and, more importantly, there were absolutely no signs in both his private and personal life, even on the day itself, that this was coming....
Whatever the experts eventually determine as the reason for the FO's actions, without him there to answer himself, it can only be a guess at best....