Reading all the comments, there are two thoughts I had regarding the killing and game difficulty.
1: Normally, an adult civilian won't automatically go around shooting people and killing them from the get go right after they get a gun. Ripley was shocked when Axel shot and killed the guy she knocked on the head. The situation might have been extreme, but if killing was meant to be a natural part of the game, there would have been a event where you HAVE to kill humans in order to get through. Personally I think the pistol is useless, and the shotgun too loud. Give me a cattle prod anytime, anywhere.
2: Game difficulty. Yes it was recommended that the game be played on Hard mode to have the experience of the Alien stalking you around, but that's not really what you see in the Alien movie that it's supposed to be based off... In the first movie, the alien only kills when someone steps into its current kill zone, or when the team starts trying to hunt it down. It moves very slowly and silently, only showing itself when it's about to make a kill. A:I on normal mode is about par with the level of appearances by the Alien, where it only stalks you semi-passively, instead of aggressively. The active hunting mode is more like the aliens from Aliens, Alien3 and A:R.