Many people who suffer depression don't have specific reasons for it, they may have specific triggers but not always. He could have a genetic predisposition, something to do with a hormonal imbalance, etc. Nobody here is going to have access to his medical files and I doubt his family is going to make them public and taint his memory even more. He was depressed, that's all your going to get.
One of the reasons he was depressed is because he had relapsed after being sober for 20 years a few years ago so the drugs and alcohol could affect that if he was on them. He also has an incredible career and he hasn't been in anything successful in a while hes still was on a lot of movies and shows but the ones he starred in didn't do to good with reviews and financially.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.