How do they know that Anna Nicole's overdose was accidental? She was severly depressed before she ped away

I think she was to the point where she really didn't care, not be rude but even if your as dumb as she was, she must have known that mixing all of those drugs would have bad consequences. She lost her son, she was being sued from all over the place, she probably had post partum depression. I also heard that in her diary she wrote down that she wanted to die so she could be with Daniel. So maybe it wasnt completely an accident, maybe it was an "if it happens , oh well" type of thing.