And she asked: "So, how do you know you aren't the bad guy?"
Well, there you have a point. You have these soldiers patrolling the corridors of a scientific installation, and then one guy comes in, thinking they're all possessed, and he starts killing 'em all! And while even the sergeants can't survive a single shot-gun blast, this guy can. How can you explain that, other than that his unholy paranoid rage pumps the adrenalin so hard that he keeps on going? Obviously they would try to shoot him on sight -- he's a madman!
Suddenly, I didn't feel so good about slaughtering everyone in the level.