So, this is gonna be kind of a strech and bare bones opinion, but i was thinking for a while now, what is the metaphor? What is Doom really about? I think i figured it out, but i could also be totally insane, i will never know for sure until i play both original games and D16 on LSD, but here i go:
I think that Doom is a journey through violence, where we fight our own violent instincts/impulses.
Think about it, zombieman is the same exact model as Doomguy/marines, but "evil", so there is our nemesis right there, the guys with the shotguns are us but with shotguns, and all the other creatures also represent ourselves/the player but in other aspects.
The thing that got me thinking about it is the Spider mastermind design and how it reflects the player (also is not a coincidence that is the final main boss of the first game and the master planner behind it all, not sure about the Icon of Sin symbolism yet):
Is a brain attached to a gun and legs, just like us when we play are our brain attached to the "gun" (the mouse) and the other hand we use to move, is like an evil reflection of us not as a character but as a player through the game, see what i am getting at?
Now i am not so sure about Doom 2 but might be more of the same, with the Earth setting having something to do with a hidden meaning in the game too.
I know it sounds like a crazy, fragmented theory, but hopefully someone will think about it and see what i am saying, is something that i wanted to say to someone for a time now and this seems like the right place.
D16 might be about the same thing, but i have to reflect on it.