Also I love City of the Living Dead even though the final shot of the movie is...bizarre.
City of the Living Dead is one of my all time favorite horror movies.
I have a theory for the ending.
The original title is
Paura nella citta dei morti viventi, and the direct English translation for that is
Fear in the City of the Living Dead.
Fear attracts the monsters. There are many scenes where things are first about normal, then people start to become cautious and afraid of something, be it a news story, sounds, or something. The more they become afraid, the closer the monsters come.
-Look at the bar scene. The people in the bar become scared of what they hear in the news and what they tell to each other, then comes the sounds, then corpses appears outside the window, then people become more scared and then the ghouls suddenly appear inside the bar.
-Look at the scene with the kid in the alley (or wherever it was). He is confronted by the zombieghost of the girl. The girl comes closer and closer as the kid becomes more and more afraid. Then the man appears and he literally closes his eyes and focuses as much as he can to stay calm and the zombieghost disappears.
-Then there's the man who steals jewellery from dead people at the morgue. He is visibly cautious, nervous and clearly afraid someone could catch him. Then he gets bit by one of the corpses.
In the ending everything is feeling ok again, the boy starts running and without anything really happening yet, the woman starts to panic and starts to become afraid, then the effect comes to the screen and shows how fear is starting to turn things bad all over again.
Not everything can be explained by this though, but that's the closest I've been able to figure out and it fits with the original Italian title. It kinda explains the ghostly behavior of the dead and you could also say the tendency of them crushing people's brains kinda fits to that theme too (fear being a brain thing).