Maybe John Doe was the devil after all?

Or Keyzer Soze for that matter (Spacey also played him in "The Usual Suspects" released the same year.)

interesting interpretation... it probably works, especialky if we consider the idea that the devil works through people and uses their moral weaknesses...

No, He's a man.

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Or David Fincher is shit and he can't make a plausible story for his dear life?

I think that’s why this film has legs - it probably has the most twisted and evil kills ever portrayed in a serial killer thriller… and yet John Doe is making a valid moral point about our spiritually vacant times, in accordance with Christian teaching, and Old Testament God wasn’t above genocide and brutality when he had a point to make.

God and the Devil are both swimming around in this fucked up film, its moral conundrum continues to haunt us, and that’s why it’s a classic.

It's almost as if it was a highly contrived fictional story and not a documentary about a real serial killer.