Think of being a marine in the Alien movies.
Think about the fact that the crew was dispensible in preservation of the "specimen"
Now, transform that into, Scientists developing similar specimens for a Corporation to profit by. Things go horribly wrong producing a myriad of "mutated experiments" that are basically capable of making any human a Zombie, just by infecting them.
It is you "Duty" as a Marine, to protect the installation and such. Which has very unstable power stations, making the WHOLE game Pitch black and not being able to see unless you have a flashlight. But to "Defend yourself" you have to switch to your weapon away from your light source.
Also, it seems that some of your fellow Marines have gotten infected as well. Proving MUCH more difficult, as you basically have to "Head shot" them.
Now, when you see this was all caused by a Corporation Obessed with making more profit... well... maybe you would understand.