Well of course I can't prove anything any more than you can but MJ's use of strong prescription medicine is out there in the media. It is alleged that he became addicted to painkillers after that awful accident when his hair got burnt and tranquilisers after the child abuse trial. I guess we will never know for sure but it seems likely to me.
Conrad Murray should never have done what he did but MJ would have found another doctor to supply him with what he wanted if he had refused. In my view this makes him responsible for himself, just as every junkie in the world is responsible for themselves. They can't blame their dealers.