The game developers said in a game article that the "canon way" to play the game is to not kill any humans at all. That being said, I killed exactly 10 humans on my 1st playthrough of the game. I didn't kill any humans I didn't have to though, if they gave me a chance to run then I took it and tried not to take advantage of them (of course, sometimes they did something stupid and fired anyway which attracted the Alien and got themselves killed), but those humans that said things like "Let's kill her" or "I'm going to GET you" were as good as dead in my book and I had no problem whacking them off whether it was by maintenance jack or other means.