It turns out that Robin Williams was suffering from early stage Parkinson’s Disease:

Patients with Parkinson’s disease often suffer from depression, medical experts said Thursday, after Robin Williams’ widow revealed that the comedian was in “early stages” of the neurological disease at the time of his apparent suicide.

The same biochemical changes in the brain that cause the hallmark physical symptoms of Parkinson’s — tremors, slowed movement, rigidity, balance loss — can also affect mood, said Dr. Jeff Bronstein, a neurologist in the Movement Disorder Program at UCLA.

“Obviously getting the diagnosis can make people depressed,” he said. “But we also know that there’s a much higher incidence of depression even before the disease is recognized. We think it’s one of the early symptoms.”

I don’t think that anyone can truly understand what was going on in his head, but the fact his limbic system was being short circuited from a lack of dopamine was certainly a major contributing factor.