The less light you have on the better you see
What did you expect the person who hit the deer to do? If he/she was in the middle of the road in heavy fog trying to drag the dead dear off the road some a-hole going 80 would have probably ran he/she over.
Thats a very good point.
OP, could you have stopped if there had been a person in the road dragging the deer off, like you thought they should have?