Optional only. Something like that would make a lot of people stop playing naturally and play to only be a certain alignment. Also many things are situational. I would hate the game to tell me something I did was one alignment when it could easily be explained as another.
There is a difference between what you do and why you do it. I think alignment speaks more to the why, which would probably be difficult to pin down. I can kill a bunch of goblins because I enjoy murder or I can do it because they are a threat to civilization - both would probably look the same to the game.
This.
