Robin William's most important planetary positions were all in water-the Ascendant Sun and Moon in Scorpio, Cancer and Pisces respectively. The water signs are feeling signs, they are highly everything passes through their feelings. Kurt Cobain was also all water signs!! But it does not mean all people with water sign are going to be susceptible to depression or addictions.
I will not bore you with details but he has the classic and most potent combination for addictions-Moon conjunct Rahu.
He was struggling for years with alcohol and drug dependancy, as his horoscope shows, due to the Demonic Rahu planet being with his Moon(emotions). This creates huge mood swings and roller coaster feelings. Rahu Moon indicates someone who has not learned to handle their emotions in past lives and in this one, emotions take over and lead to some kind of depression or bi polar behaviour. (Catherine Zeta Jones has the same combo). The consciousness is focused in the mind and not in the immediate moment, which is a recipe for the person to identify too strongly with their mental chatter and thoughts(and moods) control them, rather than the other way around. In one word: the mind becomes uncontrollable and a person is like a ship in the stormy waters of incessant thoughts and feelings.
The fastest way to switch off for a bit the incessant mental chatter and mood swings is by numbing the mind and perceptions--the easiest way is through drugs and alcohol.This is one of the most painful and difficult combination to have and to overcome --believe me, I know, I have it too.
Rahu is an insatiable influence--it is represented in astrology by a Dragon's head, without a body--all it wants to do is take more, more and more--but it never feels full or satisfied--when it is with the Moon, one is emotionally insatiable/turbulent, susceptible to depressions and addictions. The best way to overcome and control this combo is through sport, meditation or substituting the harmful addictions with healthy ones. And no matter what these ppl do--bad or good, they do it to extremes and with total obsession!
Unfortunately one is never healed from this position--so addiction can always resurface, emotional instability can also raise its head during more testing times. And why did it happen exactly now?
Because He has been going through one of the toughest transits in life--Saturn passing over the Ascendant. Saturn lowers the vibration of the whole person when it passes over the Ascendant (the self)...once every 29 years--negative thoughts and feelings take over--and if someone has already weaker emotional stability....unfortunately they can do some extreme things...