Meperidine, the active ingredient in Demerol is an opiate, used to treat moderate to severe pain, however, as opiates go, it is actually a relatively mild one.
Extremely high doses can cause difficulty breathing, respiratory depression, somnolence and even death.
That said, I saw the list of medications he was on, in total there were 4 narcotics and 4 sedatives, thus, it is doubtful that any single drug killed him, it was probably a combination of all of them.
The most dangerous that he was on was the Diprivan, which IS NEVER given for unsupervised home use, it is ONLY approved to be used in a clinical setting where the patient is under CONSTANT monitoring by medical professionals.
When you take many drugs at once, your body is unable to process them and usually goes into shock, resulting in a shut down, without prompt medical attention, death can very quickly result.
Are there any other questions I can answer for you?