I guess that to understand the meaning of the song deeper, I would need to step out of my belief about Michael being some kind of "voice healer".
What I see then is a great artistic piece, where the song becomes tridimensional.
When singing Morphine, the sound, the voice, the music and the lyrics represent an extreme state of mind. Morphine analgesics are used to relieve severe to agonizing pain and suffering.
Demerol is used to treat moderate to severe pain
When he sings about morphine, his pain is extreme, all the unjustice and comments and critics about him.