If the civilians were terrorists it would all be okay
Except the terrorists would probably be shooting back? If they are terrorists, they aren't exactly innocent are they?
Why are people comparing this to other shooter games, especially military types? In almost any other game, you're killing a person who has the intent to kill you first, for whatever self-righteous reason. With soldier type games, terrorists want to kill you because of some religious agenda or some stuff. Not because you are shooting them, because you don't fall in what they view as correct or some stuff. You are defending yourself and other from harm. Aliens? Well they obviously want to end humanity, better sit back and let them! It's not senseless killing, it's justified defense.
This game? You are those terrorists. You are the genocidal alien. Except 90% of your enemies aren't armed and probably don't wish ill towards you, or don't even know you.