Michael wanted to sleep. Desperately, so he could get through rehearsals but also to get through his life. Michael was using it in the 90's too. He could have been killed then.
If you want to call him an uncontrollable idiot, then that's your thing. I just try to understand him, and from what I understand someone pointed him to Propofol to help him 'sleep'. We all know propofol is not something you can just 'get' for use at home.....so Michael found a way to get it, through Murray (who should have told Michael to stay away from that ish instead of injecting it into his body and eventually killing him with it) who sadly did "anything for money".