I tend to think she accidentally OD'ed. There was some special on her not too long ago that basically showed that is was possible that she took too many pills without realizing it and that they had plenty of time to dissolve, etc. I think she probably did screw the Kennedy brothers, but if she was going to talk, I think she would've been dismissed as a screwy, scorned woman.
I think people want there to be this huge reason for her death because her final moments weren't extraordinary. She may have had an extraordinary life, but her end came like so many who are depressed and taking one too many drugs. I'm a fan of hers and I think she was brilliant (still do), but she was one of those people who just had way too much crap handed to her in life.