[TheDragon]

With some quick Googling:

Oxycodone is used as a pain reliever.
Hydrocodone is also a pain reliever and anti-inflammatory.
Diazepam is used to treat anxiety disorders. (Shakiness, agitation, etc.)
Alprazolam is also used to treat anxiety.
Temazepam is used to treat insomnia.
Doxylamine is used to treat allergies.

I highly doubt that he was aware that these specific chemicals produced a fatal combination, myself. If it was deliberate, you'd think the coroner would say he found "extraordinary" levels of x chemical in his system that overloaded x organ. Otherwise, he'd probably have to be a chemist or a doctor to realize what he was doing. But that's just me.