I agree. It's way too short for us to really get into the atmosphere. You might try actually describing things, like the hallway he was in at the end of this part. What color were the walls? Were they splatted with blood from the corpses? Were the corpses human? Were they soldiers, cheerleaders, bigfoot? And you might try describing what the character feels, whether it's afraid, mad, whatever. Describe creatures, too. I kind of took me awhile to figure out that the first monster was a Cacodemon. At first, I though it was a Lost Soul. This is but a suggestion, but I would describe as something like this:
"A floating monstrosity, large and pink, rounded a corner, and noticing him, floated towards him. Its one large, green eye took him in, as if weighing him, trying to ascertain what threat he posed. Finding no obvious problem, it emitted a high pitched screech, like nails on a chalkboard, opened its mouth and hurled a purplish, luminous ball of electricity towards him. He lunged frantically out of the way, bringing his rifle to bear and squeezing off six rounds in quick succession. The bullets all hit their marks, and the thing deflated like a baloon, its eye falling out of the socket as a river of blood flowed freely from its wounds. He complimented himself on a job well done, and proceeded down the hall from which the thing came."
See? I think that is a lot more descriptive and atmospheric. It draws the reader into the story, giving him/her details about the character and his foes. I don't mean to completely revise your already wonderful story and come off as some kind of English Teacher snob, but you asked for comments, and it was comments you got. I'm glad i could at least try to help.