I think it is an opium dream. He's a man who betrayed his friends, and had nothing left, but to become a junkie. An interesting note is that opium supposedly makes you hallucinate of the future, which is why many of these characters could be just in a messy dream. Maybe Debbie didn't get together with Max, maybe it was an incoherent, guilty dream. Both were betrayed too, one by a tip, the other, rape.

It's whatever you want to think of it. I would've loved to hear a Leone commentary if he was alive.