Applicability: It's common courtesy to write a review of this fanfic as if it's a literary classic studied in English classes.
Fridge Brilliance: The collapsing ceiling that traps John can actually happen in the original DOOM games; some crushing ceilings do not retract upward, and if the player survives the crushing part he will be immobilized inside the architecture.
DOOM includes a piece of lore that says the demons collapsed a whole temple on the Doomguy to keep him contained. They may well have used some demons as bait to lure him into the area, and deliberately made the ceiling fall on him. The And Then John Was a Zombie that followed was Hell's attempt to corrupt the Doomguy, which ultimately failed.
Hilarious in Hindsight: A soldier named John, trapped in a losing battle that is taking a toll on his sanity, ends up finding out from what he thinks is the voice of his commanding officer over the radio that he indeed is the true monster. This is hilariously similar to the infamous Twist Ending of Spec Ops: The Line a few years later.
DOOM's backstory strongly implies that the trips to hell and back have made the Doomguy at least partly demonic.