This 4 year old interview with Williams is very revealing--particularly about the fear and anxiety "about everything" he experienced at times, as well as the vulnerability he felt as the result of his open heart surgery. The interviewer described him as seeming somber and "mournful".
http://www.theguardian.com/film/2010/sep/20/robin-williams-worlds-greatest-dad-alcohol-drugs
It's not difficult to imagine how a diagnosis of Parkinson's might have affected an already emotionally vulnerable individual--it's a very tough diagnosis for anyone to receive--and his anxiety and depression about what the future might hold for him, and for his loved ones, may well have been overwhelming. We don't know how long he had been suffering from Parkinson's, or in what ways it had already affected him, or his fears about it becoming public.
We'll likely never know exactly why he decided to end his life now, although he may have feared that if he lost his grip on his sobriety, he would seriously imperil the quality of life he did have left, and might lose what mattered most to him--the love and respect of his family--and he may have felt he was losing that battle with the bottle. Maybe choosing to die was his way of taking control, or taking the only option he saw as offering control. Maybe his suicide was done on impulse, in response to overwhelming anxiety and depression he just had to escape from. I don't think we'll ever know.
It's just very sad that, this man who brought such laughter to others, had such a tragic end.
But, it's clear in that 4 year old interview, linked to above, he did have a dark side that he often concealed with his manic humor.
