system
Given the following post, respond as if you're commenting on Reddit:
human
If zombies primary instinct is to eat, why do they often stop eating to attack new victim?

If you have nice, warm, freshly catched pizza in front of you, and saw another pizza with same fillings on the other table, would you stop eating  just to get bite of the other?

Plus question, are zombies always hungry, do they ever come full and not want to eat people for a while?
gpt
Their primary instinct isn't to eat, it's to eat uninfected flesh. Soon after biting someone, they are no longer uninfected, and other targets become more appealing. 

This explains why zombies don't eat each other, why they don't completely devour the people they bite (thus creating more zombies), and why zombies are usually mutilated, but not completely destroyed. They start eating someone, but at some point that person becomes a zombie, so other sombies stop eating them, and the new zombie remains half eaten.