As someone raised by a bipolar parent, I know exactly what you speak of. I watched countless times as my mother blew through money, maxing out credit cards, on silly, worthless crap only to crash when her manic stage passed and become terribly ashamed and depressed.
I believe Williams was clinically bipolar. Just watching him perform is testament to this. Now that he has taken his life, anecdotes attributed to classic bipolar personality are surfacing from everyone who knew him.
People ask, "how could he be depressed when he was so rich, successful and famous?" His brain wasn't wired right, that's how and asking this question is just as absurd as asking "how could he get cancer when he was so rich, successful and famous?"
True depression isn't just being down in the dumps. It's not just being sad. It is a clinical illness, just like cancer, just like diabetes and it can affect anyone regardless of fame, wealth or success. I think the one silver lining of Williams' suicide is that his death may bring attention and awareness to the very real ailment of manic depression and bipolar personality.