Hey, this is a funny yet interesting concept. The elevator music breaks the intensity and gave me a chuckle.
I think it can benefit from some faster pacing, I like the book and the candle but wasn't sure what its purpose was. Maybe the book can be your UI, like hold the counts of raw and processed materials.
While the zombies/corpses feel good to shoot at, I think skeletons vs a gun doesn't feel good. I would introduce melee with some impactful swings of a graveyard shovel.
I can see a lot of potential, like instead of the WW1 feel why not make weapons from the flesh and the bones, give it more purpose. Like a "flesh gun that shoots bone darts" - rough idea, but I did enjoy the demo!
Good luck with the rest of your development.