Interesting. I was wanting some type of an antagonist. But not one that is generated from actions. The way the antagonist you are describing is along the lines of what I want. A creepy element conjured up from the main character.
I like it.
If your following with the little girl, make her a childhood friend that evolves into his love interest and first kill
Or his younger sister or daughter that he killed : \
Also if your going with the transition to creepy have one of the levels start off with the protagonist walking through a city and then going up an apartment building because he saw the little girl at the top. As he ascends the building the areas become more dimly lit, shadows streak across the walls. Nothing ever pops out at you until you reach the end of the final floor where the walls ooze blood, broken childrens toys lay across the floor, the area is almost pitch black but you can see the ethereal form of a child or walking past you very briefly. When you reach the end you find a dead end, and when you look behind you the girl stands there staring at you.
Not so much scary as it is for the environmental feel