It is sad about Robin Williams, but there seems a long history between comedians and mental health problems of one sort or another.
You must be crazy to start with to want to get up in front of a crowd and try to make them laugh, surely the most difficult thing in the world to do, but thank god they do it.But in private many of them seem to be introverts, with a dislike of, or trouble with people ( again, perhaps not so crazy) but maybe that's too much of a cliche.