I don't kill anything that doesn't try - and fail horribly - to kill me first. I generally either sneak past hostiles or knock them unconscious if the game permits. Metal Gear is a good example, no one dies, they all just come down with a sudden case of narcolepsy.
There is the occasional unlucky bastard who falls face first onto a live claymore after being knocked out but that's not my fault.
Once someone starts trying to kill me though, be it a man, woman, child or rabid hamster I have no problem killing them.