Like it or not, there will be much debate on this topic. It is what it is, yet another mystery of Rock and Roll. And if you're into that sort of thing, I'd recommend this book: Amazon.com: Take a Walk on the Dark Side: Rock and Roll Myths, Legends, and Curses: Books: R. Gary Patterson

It starts with Robert Johnson and works its way through the history of rock. And needless to say, it includes Kurt Cobain