It’s blatantly obvious as to why someone would write “God Is Dead” on a wall in the middle of a crazy ass zombie apocalypse, but it’s pretty weird how extreme of lengths you are going to to understand it.
Say you are stuck in a zombie apocalypse, and I don’t mean a “normal” one. I mean a crazy ass one with super zombies and ones that run. Would it not be a logical conclusion that the still living religious individuals would begin to question their faith? It was obviously just meant as a bit of flavor to the world, not some sort of controversial political statement being pushed by the game or one of the member’s of it’s development team.
There is no need to read into it.