Ok, if there is any way this happened accidentally, I think I've come up with a likely scenario. The article says he slipped. He has two nails through the right side of his skull and four through the back.
Possibly, somehow, as he fell, the gun or his arm caught on something so that it was positioned behind his head. Now to actually get the gun to fire six times in rapid succession seems complicated, but I have seen a gun recoil after firing into a hard surface. Now if he is holding the gun loosely, it is possible that the gun recoiled, lifting itself far enough off his head to reactivate the safety. Gravity then caused the gun to fall back against his head, firing again and recoiling again due to the hardness of his skull. This happened 4 times before his head slumped over when he fired 2 to the side of his skull as the gun slipped off. As you can see in the x-ray, the 2 on the side were much less accurate.
However, that seems like an incredibly impossible sequence anyhow, and I still can't imagine that that isn't the result of foul play.