No longer a game

It started as a game. I mean it WAS just a game. I was playing a computer game, but then... It's funny how fear of death can warp your mind and make you forget things that happened before that fear. Things that you feared before the death.
It was just a video game. I was pressing "F" on the keyboard to highlight objects, that I can interact with. Objects, that told me the backstory of the house I and a few other characters were investigating. Or maybe it was a castle? It was pretty big inside at least. Why were we there? What were we investigating? What happened here? It's just a blur now. It's inconsequential. What matters is...
It's no longer a game.
At some point it switched from third-person to first-person. I did not bother, I thought it was just part of the gameplay. Maybe a way to make the story feel more personal. Interface was still there, I still got flying text with tip to press "F" to highlight objects of interest. And I did. At least I though I did, because as expected some things got highlighted by a green overhaul. And I did press "E" to interact with them. And there were some comments from and for those objects... Something important about what was going on, but... I can't remember.
But I can remember the feeling of being watched. Feeling of something breathing down my neck. And then I was sent running. The floor started falling away into a sea leaving just a small path near the outer wall, near the battered windows. I ran to that wall with only one thought: I can't afford to fall into the water or I will die". And soon I saw why.
A predatory fish, like one of those huge ones, which lived along with dinosaurs, was emerging from the dark depths of the killer-water. It was trying to bite my legs, but I ran to avoid that. I... Did I threw someone into that water? Someone who got snapped in two by that fish? Was it a person? Or did I imagine it?
There was no interface now, but I did see some green blur over some objects. Over some boxes and a bookcase and an air-vent. I felt claustrophobic in it with my yet nameless partner in it. Was there someone else besides us there? Did it bit me?
Out through the vent we went, searching for a moment of a breather, but even when outside we could not relax. Roms were running through our paths - showers, meeting rooms... High voltage sparks in some, reaching their blueish tentacles, trying to get us. Some strange people wearing white coats in the distance, standing there as if paralyzed, as if..
Zombies. Lots of zombies. Hoards of zombies. Through another vent and out on a trembling roof. If you could call that metal net a roof. Zombies ahead to as far as the eye could see. One of the survivors yelled at us to prepare for a fight. He told us that there was an accident yesterday and now because of it the fence won't hold much longer.
I felt that fence, supporting the "roof" give in. At the corner of the eye, as I was falling to the ground I saw zombies as big as a dozen of men. They were huge as trolls. Maybe they were trolls? I also saw zombies with heads splitting like grotesque flowers, showing off hundreds of sharp teeth. They were fast, survivors were barely able to deflect their attacks with bullets.
One ugly zombie with yellowish skin covered in pimples with a few nails in the head is running at me as a psycho. His fingers end in long sharp blades, which I stop millimeters from my throat, as I grab them with us tumbling down to the ground. This thing has no eyes and is scarred all over with scars making its skin look like that of a reptile.
I have no weapon. All I can do is smash its head with bare hands, while holding its far from myself. I hit it again and again and again... I get a chance to topple it over and then I grab its head and smash it against the ground. And then again and again, until there is greenish goo coming out the back of its head.
I hear a thunder-like growl from one of the "trolls". I see that there is a hole in its stomach, probably made by one of the bullets. Some goo is sipping through it. But then it begins o heal quickly and new bullets are ricocheting from it. I hear a hissing sound and turn around: the zombie I just killed is slowly standing up, facing me. It lunges at me and I barely dodge. It does not attack, as if calculating something in its head, evaluating me.
"We need to kill the small and the big ones simultaneously!" I yell at the top of my lungs, hoping someone will hear me. Another dance with the zombie and its down again. But the troll is standing tall. Maybe it's the wrong pair?
After a third fight I fall to my knees. I am dizzy, I feel like I will pass out shortly. This is not just the walking dead. There is something more horrible behind all this. It's like the whole world is dead and tries to do the same to all even remotely living.
This was just a game... Just some investigation... How did it turn into an nightmare I can't possibly survive?